If you can spare one of your two PCI slots, apparently the Sonnet 7200 G3 works - and they're knocking them out for $19 for 400Mhz these days.

I say apparently, because ISTR someone here tested it. A bit of searching might turn it up.

The L2 cache slot G3s do turn up on ebay with some regularity, and they tend not to be too expensive