I got mine off my school's online resources - that being the case, I'd say if you can get them this way, they all worth doing (I mean, free stuff is always worth it). Assuming you don't have something similar, either in hard copies (your school's library, maybe?) or at all - itute offer some for free,
and some careful googling might point you toward something good too.Pretty sure if you asked a teacher, they might be able to organise some exams to be purchased. I haven't done very many yet, but I find heffernan usually leans toward too simple or pretty irrelevant - insight's been pretty good, and I also liked TSSM. It's probably a matter of preference, though.

Edit: As for where to purchase if you do it independant of your school, here's some links that might be useful:
HeffernanTSSMInsightKilbahaLisachemMAVNeapthere's more than that
but I'm actually pretty lazy