There is fairly low-tech hack on aminet that gives the 500 a single Zorro II slot. Don`t see why you can`t plug a X-Surf or Ariadne into it.
Although to be honest, using the net on an A500 isn`t going to be fun. For mail, news etc it`ll just about do. But web browsing is gonna be dithered to hell and back in the 500`s low colour screenmodes.