I use an EtherLink III with mine, plus the hacked Farallon driver I maintain. Other 3C589 cards may be compatible. (The page is a bit old: my 1400 has a G3/466 in it now.)

http://www.floodgap.com/retrotech/mac/enet3c589/

RAM is very hard to find for 1400s. Unfortunately, yes, I agree with everyone else that you're better off finding a parts 1400 and raiding it.