Is it normal for a PhD program to retract or delay financial offer for international student? I am an international student and about a week ago emailed my top-choice PI about the current status of my PhD application (top-50 university in USA). He said, that he is going to offer me a position with full funding via "graduate student researcher" assistantship.

Next day I have received a letter of acceptance. The email says "we are unable to guarantee you financial support at this time", blah - blah, please contact your department. I have contacted dept and PI, they say everything is OK, my program offers funding for everyone, I will get stipend and GSRship with full funding. Ok, I have accepted this offer officially. 

But until now they refuse to tell me my exact salary and now i do not have any financial certificate, I do not know my GSR appointment, I do not know anything. I asked them to give me any financial document because I need one to apply for my visa, but instead they tell "you will get about 1,5-2k$ stipend, everything will be ok". 

Ok, now I have to apply for I-20 and I can not do this because I do not have financial certificate for my first year in the program. The grad coordinator does not answer my email. 

So, is it normal? What should I do? I am just waiting like an idiot because I even can't find housing (because I do not know anything about my funding)? I have rejected another offer (with official TA contract, which was attached to the acceptance letter). As I believe, they can just tell me something like "oops, we don't have funding offer for you" and I will search for another job outside academia.
Many departments guarantee that their students will be funded got a given period of time, although they can't provide specific details on where the money will come from in advance. That's probably what's going on here; they can't issue the letter yet because they don't know what they should write.

If you can't afford to wait to apply for the visa, then you should contact the international office at the school you'll be attending, as Bill Barth suggests in his comment. They should have plenty of experience issuing such documentation, or helping incoming students to get the information they need for their applications.