I'm guessing your CPU is a big Cartridge? If it is then a Slot 1 Pentium 3 would be possible, without a Motherboard upgrade. You can also get adapters to allow Socket 370 Processors in a Slot 1 Mothreboard.
http://www.kikumaru.com/pc/s370tos1/index_e.html (list of Adapter Cards at the bottom)
Good place to look for the Cards/CPU's is eBay or your local FreeAds (newspaper adverts of things for sale).