A Pentium heatsink from an old PC will probably fit. Many 486 heatsinks will do, if a little bit small though.
If you have trouble fitting the heatsink over the CPU, then a Dremel might be usefull for cutting the heatsink. Just don't damage the surface that will be touching the CPU as it is important that the heatsink have full contact with the CPU, especially at the center, to function properly.