Your school should provide at least one practice test from College Board for you, administered from the school itself. They give you a packet that gives you information about the PSAT & NMSQT, tips, and a full-length PSAT test.
Truthfully, those tests are going to be the most reliable, so you should try and get one beforehand if you haven't already. As far as free practice problems go, try: PSAT/NMSQT: Preparing PSAT Test Practice Questions - Help your PSAT Exam Score with free PSAT Test Preparation
(the main problem with the second link is the fact that they don't really explain their answers, so if you got it wrong and you weren't why)
In any case, I'd suggest not to do much more than take a diagnostic PSAT test and maybe review a couple tips and tricks for tonight/tomorrow night. I've heard that cramming and getting bogged down by overload could do more damage than help for the PSAT (I am also taking the PSAT this Wednesday).