Ethernet Cards can be found but you have to look and a little luck is required.  For the SE/30, the MacCon ethernet card is shared with the Mac IIsi model as Trash80 stated above.  Be on the look out for an SE/30 or IIsi with a network card in it.  I just picked up this little gem myself and got a IIsi in the deal http://www.ebay.ca/itm/251961542155