I've got a Nvidia GTS-450 graphics card and use it since many years, but the past year its fan got broken and then I had to attach an emergency fan. I searched a lot online about a replacement but I didn't find the exact one and the original fan is discontinued. Tired of the "emergency fan" I decided to convert it into an "original" fan.
Step 1: Check the Fan Size
This sounds very logical, but this is very important if you don't want to cut that beautiful case/cover of your graphics card.
Step 2: Adapt Your "new" Fan
Got the right size? Now cut the fan holder like on this photo. Don't cut the cable when cutting it's holder!
I did that using a knife.
Step 3: Fix Time!
Unscrew and remove the case of you GPU and select your preferred method to attach the fan to the cover. In my case I used a cheap glue because I had nothing better at that moment. It is important to press and hold the both elements for a while to be sure they glue correcly.
This is not a very recommended method because the GPU can reach very hot tempreatures inside and a wrong glue can weaken with heat.
Once the case and the fan are tightly joined, screw the whole case into the dissipator (this can vary a lot between models and brands) and you are ready to mount your restored grpahics cards into your PC!
Step 4: Powering Your Fan
Because this fan isn't designed specifically for your GPU, it's possible that it moves less air. It is better to connect it to a Molex or fan pin on the motherboard. If your fan doesn't have any speed reculator it will spin at maximum speed, but if it does you can use specific software like SpeedFan to control its speed as you want.
Step 5: Enjoy!
Now you can play again the games you wanted or to do whatever you want without worrying about overheating your GPU!