Is there a good method for sourcing a TAG chip? I'm looking at pictures of other 486 boards like the 486SPM and seeing if they have different chips, which some do, but the only source of them would probably be fakes from China.
Unfortunately, there's really no good source. TAG chips aren't documented well, so you'll have to do a lot of research on what part you need, based on what amount of cache you want installed. The best thing right now would be to remove all of the cache, because it won't work at all without a TAG chip. The only place I could recommend that has a better chance of not having fake parts is utsource.net. They deal in obscure and old chip stock.
Based on the motherboard diagram, you'll either need a TAG chip with 8kx8 for 128k of L2, or 32kx8 for either 256 or 512k of L2 cache.