I'm in need of a 56k modem to put in a B&W tower. Anyway - I'd prefer to trade. They seem to to go for $15-30 from a few online shops soa cheap trade would be suitable. Maybe you need some PC100/133 RAM? A component from another Mac?
A CPU upgrade wouldn't hurt either, but it's much less critical. It's a 400MHz G3, IIRC.
I've got a modem from a Yosemite - it's yours for $20 shipped. Got PayPal?