check out www.guitarfetish.com, I have used their parts and like them. I have a Strat wannabe and bought a new trem block and saddles for it, man did it change its sustain and tone. Was a good upgrade. I also installed a graphtech nut but that's another story.

Also stew-mac.com has plenty of info and parts

I am curious on why you want to change it, I also have a Fender Stratocaster and am quite happy with the trem, there are other things you can do to improve it other than replacing it. Most "problems" with the trem is improper adjustment. Come on over to www.strat-talk.com, lots of fender questions and answers there.