eBay is about the only place I can think of - don't get anything older than Panther though as the older versions of OS X are slow, buggy, and almost as bad as OS9 for modern software compatibility, especially 10.0 and 10.1. Personally I highly recommend Tiger for G3s. By the way, it should have built-in ethernet, though it could be disabled on the limited access account that you seem to have.