I'd highly recommend that you look at the following roundup (link below). They've compared 393 fans of various sizes and have picked out the best ones for you to look at.
http://uk.hardware.info/reviews/2614/393-8092120140mm-case-fans-reviewed
Since you're using this to replace the fan on your heatsink, you'll want a good amount of static pressure. If you don't mind the price and noise, the Scythe Gentle Typhoon AP-31 (high speed Gentle Typhoon) costs around $22 but pushes a lot of air and should generate a lot of static pressure.
Close behind you have the Scythe Slip Stream 140mm 1700rpm (SM1425SL12H).
It has 120mm mounting hole positions so it should work with your heatsink. Sidewinder Computers has it for around $11-$12.
The Scythe Slip Stream 120mm 1900rpm (SY1225SL12SH) is a good fan too. It costs around $7-$8 at Sidewinder. Newegg has it for $17-$18 (and I think you have to pay for shipping...probably not. Still, that's a ridiculous price).

Cheers,
Hybrid