You could probably find a stock Intel HSF on ebay or by posting a "WTB" add here at AT but the best way to do this is to get yourself one of those $5
GC-68's from SVC.com.
These are an ablsolutely great cooler for the money. They'll cool your PIII CPU better than the retail HSF and be even quieter while they're doing it. I use them an almost all of my PIII builds. They snap right on to the CPU socket just like the Intel HSF so you'll be up and running in a flash.
Order yourself a $4 tube of Arctic Silver Ceramique while you're at SVC. You'll need some paste for you new heatsink anyway and this is some of the best you can get.