What!? You couldn't be more wrong saint19. Dunno where you got that idea from.
I install ALL my heatsinks onto the CPU while the motherboard is in the case. I've never once HAD to remove the motherboard first, but have some a couple of times to do a better job of sorting the fan's wires out. AM2 / AM3 use a simple retention bracket clip which granted is slightly easer to install outside of a case, but is still only a 2 minute job if the motherboard is installed even in tiny cases.
Back on topic.. you can find some cheap sinks here:http://www.ebuyer.com/product/198975http://www.ebuyer.com/product/176157http://www.ebuyer.com/product/147097http://www.ebuyer.com/product/169197http://www.ebuyer.com/product/164454http://www.ebuyer.com/product/195684
..and as with anything.. you pay for what you get in both quality and performance.
For what you're after, I'd spend around £20 to make sure you have a good quality cooler.