Frye's stores locally, aka outpost.com on the web.
Best Buy and Compusa charge an arm and a leg for them.
Now that that's out of the way. Your CPU temp should not be going up unless the room temp is going up. If the room temp is not going up you need to do maintenance:
Clean the dust out of the power supply and the CPU and video card fans. Use "Compressed air" (actually it's a toxic gas, not air, so do it in the garage or out doors).
If that doesn't help, and you've had the heatsink on a long time the heatsink paste may be dried out. This happens slower to the better quality pastes but affects them all. The good pastses still hold their conductivity pretty good even after dried out.
Search at osnn there are several threads on pastes and how to clean and replace it without trashing your CPU.