I wouldn't go with ISA option. AFAIK it will be slow and problematic at best
With same effort to find bridgeboard, you probably can find old Zorro networkcard for Amiga. There's lots of different models from the past and they all should work with A2000. They should be pretty cheap nowadays as they don't work as fast as PCI solutions and doesn't offer all the luxury X-surf does. So, I guess they aren't that wanted anymore. Some models don't have RJ-45 connector, but transceivers for AUI connector -> RJ-45 can be find from recycling centers for free. But there are cards containing RJ-45 connector(s) too.
