Asante and Farallon made several types of LocalTalk to ethernet adapter. Some only provide the printer functions of AFP, some will do IP and AppleTalk and some do MacIP and AppleTalk. Finding the middle one, which is what you need, may be as tricky as finding an SE/30 ethernet card.

SE/30 ethernet cards are easier to find than you might expect. A bunch of NOS cards from a US warehouse were sold off five years ago, so there are many out there.