The PCI slot fans are primarily for moving hot air out of a dead space. If you want to cool your GPU, you will have to replace the current fan. A PCI slot fan won't actually be blowing on the GPU anyway--since the dead fan is in the way. If I understand correctly, what you're saying is that the GPU fan and the RAM heatsinks are of one piece. You don't actually need the RAM sinks unless you're overclocking, so those can be permanently removed without causing anything to overheat.
There might be screws attaching the heatsink asembly to the board. Plus you will have to carefully detach the heatsink from the GPU with a razor blade, because it's stuck on with adhesive thermal grease. Use a cotton swab and rubbing alcohol to clean off the GPU before attaching its replacement.
Removing the videocard heatsink assembly and cooling the GPU with a PCI slot fan will not be sufficient.
Here are some video card fans.
Edit: Also keep in mind that your local computer store or Best Buy probably stocks these replacement fans as well.