How to get a "teaching letter" when I don't want my department to know I'm looking for a job? I am currently a tenure track faculty in a college and I plan to look for a position somewhere else. A problem is that, I can not get a teaching letter from my department (in fact I would like to keep my search confidential). Teaching letters are required in my field and I am wondering what should I do in this case? Thank you for your suggestions in advance.
As noted in my response to @strongbad, I recommend against being open about job searches if you're in a T-T position. You don't have to look far to find a lot of horror stories about chairs and senior faculty making life difficult for the 'uppity' young faculty member "who thinks s/he is too good for this school." Best to keep plans close to the chest until you're at a more definitive place in a search.

So that being said...

First, if you have a trusted friend at a nearby college who is in the same or related discipline, you might ask them to do an observation and evaluation of your teaching. 

Second, your college's teaching center may be able to do an evaluation but this involves you trusting them not to spill the beans.

Finally, if this is a middle or senior position (i.e. Associate or Full) you can ask in your cover letter that because you the need for discretion because you are already in a tenure-track position, that the request letters be delayed until you make the long short list. This at least buys you time and puts you in a difficult situation only if you're a serious candidate. 

I've seen this done and it's not an unusual request. You can suggest that you will provide your full teaching evaluations and portfolio in lieu of a letter until you make the long-short list.