Considering you have a HD interface connected to the side port rules out the possibility obtaining one of the very rare A500 specific Ethernet cards, unless you purchase a HD controller that fits on your motherboard. What you could try is making a Zorro slot adaptor with a passthrough for your HD controller. This would allow you to obtain a Zorro Ethernet card and use that to network your A500.
For information on how to build an adaptor, check Aminet or search with Google.
