A CPU heatsink from a Pentium 1 from 1995 would probably be OK. The stuff you get on modern processors would be massively oversized.
Do the calculations on wattage and degrees-per-watt. Look up a few heatsinks online on your favourite shopping site to get some representative numbers. Then just cut a piece of aluminum about that big.
I did once have a project that needed a heatsink. The required heatsink was very small - it only cost 85 cents. However it was smaller than an (Australian) 50-cent coin. So I drilled a hole in a coin and made my own heatsink, saving 35 cents.