I found an interesting quote in the thaivisa forum. It's from the late E-Sangha, written by Dhammanando Bhikkhu about the ordination possibilities in Thailand.
I have lately been getting quite a few PM's enquiring about the practical details of ordaining as a bhikkhu in Thailand. Rather than replying to each separately I will just post to this thread and henceforth direct enquirers to it.
It used to be the case that foreigners could get ordained in Thailand very easily, indeed almost at the drop of a hat, but owing to abuse of the system (e.g., hippies getting ordained just so that they could get a long-term visa) new regulations were introduced that made bhikkhu ordination somewhat more difficult.
A non-Thai who wishes to ordain in Thailand and stay here long-term now needs to enter the country with a special "monk-to-be" visa. Strictly speaking, abbots are prohibited to ordain a foreigner who does not have one of these visas. In practice a lot of abbots outside Bangkok and the larger cities are ignorant of this rule (or else they know about it, but don't give a ######!) and will ordain foreigners who don't have it. However, if you ordain in this way it's likely that you'll run into problems when you apply for a visa extension. Therefore it's best to do things by the book.
To be eligible for a monk-to-be visa you will need to obtain a letter from an abbot in Thailand certifying that he is willing to ordain you. You will also need a letter of sponsorship from a Thai layman. (At some embassies it is sufficient to give the layman's name and address). This person will be making quite a big commitment, for he will be responsible for your behaviour and for repatriating you if you go insane, commit a crime or whatever.
So, given these new regulations, there are two ways that one can proceed. The better course, imo, is to come out to Thailand on a normal visa and spend a few months travelling about, visiting temples, going on retreats, questioning ajahns, making acquaintances etc., until you find some place or teacher that clicks with you. Then notify the abbot of your wish to ordain and follow whatever procedures are in place there. These will vary a lot; some abbots may write you a letter and find a lay sponsor for you straight away; if that happens then you just need to go to Laos or Malaysia, get the special visa, re-enter Thailand and you might be a bhikkhu by the end of the week. Other abbots will expect you to go through some kind of program, e.g., spending so many months as an 8-precept layman, then so many months as a samanera, before being eligible for bhikkhu ordination. If that's the case then you might need to enter and leave the country several times during your training, as the normal visas only last for 2-3 months.
Another way to proceed is to start attending a Thai temple in your home country and befriending the monks there. If they like you and trust that you're sincere they may be willing to arrange for an abbot in Thailand to issue a letter and find you a sponsor. I wouldn't myself recommend this procedure, however, for it has the drawback that you'll be committing yourself in advance to ordaining at a temple and with an abbot that you know nothing about. (Bear in mind that once ordained, your preceptor can insist that you stay with him for five years, so you really ought to select the man with some care). On the other hand, if you are only planning for a temporary ordination this might be the better way to go.
Once you have been ordained as a bhikkhu you will be eligible to apply for a one-year visa. This can be extended every year without needing to leave the country.
Regarding the best place to ordain for foreigners, this is a matter on which opinions will differ (and sometimes heatedly!). Obviously if you already have some faith in one or another of the Theravada sub-traditions (e.g., Mahasi-style vipassana, the forest tradition of Ajahn Mun or its off-shoot, the Ajahn Chah tradition, or Ajahn Buddhadasa, or Ajahn Naeb or whatever) then the choice will be dictated by that. For example, to train with Ajahn Maha Boowa you'll need to be ordained in the Dhammayuttika Nikaya; to be a monk in the Ajahn Chah tradition you'll probably need to start at Wat Pa Nanachat, etc. On the other hand, if you don't have any such prior commitment, then my own suggestion is that you start off at some place where you'll be trained properly in Vinaya. As far as I know this really cuts down the choices to three:
1) A Dhammayuttika Nikaya temple. In general the Vinaya observance is stricter in this Nikaya than in the Mahanikaya. The drawbacks, however, are that as a Dhammayutt monk you'll miss out on much of the richness of Thai Buddhism, you'll only be able to stay at about 5% of the temples in Thailand, Dhammayutt Abhidhamma scholarship is poor, and the one and only practice tradition is that of Ajahn Mun, with its eternalist doctrine of the "citta that lives for ever." Also the Vinaya observance tends in some respects to be pharisaical rather than virtuous; e.g., the Dhammayutts take pride in not using money, but in fact most of them do have bank accounts, even including some of the Ajahn Mun forest monks. They differ from money-using Mahanikaya monks only in that they don't physically handle the money.
2) Wat Pa Nanchat. This is Ajahn Cha's branch wat for training western monks. The Vinaya training in this wat is quite thorough and not so prone to Dhammayutt-style hypocrisy. The drawback is that you're not likely to learn very much Dhamma or to get competent guidance in meditation. Also, I don't think it's very healthy to be living with other western monks during one's formative years of training; too much time gets wasted on gossiping and squabbling.
3) Wat Tha Ma O; this is the Burmese monastery in Lampang of my own Pali teacher Sayadaw Dhammananda. Though the wat is primarily a Pali and Abhidhamma study centre, the sayadaw is also a meditation master and he gives his monks at least as good a Vinaya training as they'd get at Wat Pa Nanachat, but with much else besides. This is nowadays the only place in Thailand that I can wholeheartedly recommend for a western would-be bhikkhu.
Well, that's all I can think of for now. If you have any questions on this matter I'd prefer that they were posted here rather than sent by PM, unless they concern a matter that really needs to be kept private.
I hope this will be useful for somebody (at least for me it was useful).