Below is a list of IBM part numbers that you can look for:
IBM Credit Card Adapter Ethernet I
0933280 (10Base2 card+MAM)
0933290 (10BaseT card+MAM)
0933270 (10Base2 card)
0933270 (10BaseT card)
0933263 (10base2 MAM)
??????? (10BaseT MAM)
0933495 (10Base2 firmware?)
0934156 (10Base2 manual?)
IBM Credit Card Adapter Ethernet II
0934330 (10Base2 card+MAM OE2MBSE)
0934331 (10BaseT card+MAM OETMBSE)
42H4916 (10Base2 card)
42H4916 (10BaseT card)
0933579 (10Base2 MAM)
0934216 (10BaseT MAM)
92G9319 (10Base2 MAM boxed)
92G9320 (10BaseT MAM boxed)
0934214 (10Base2 firmware?)
Note that some people have reported mixed experiences at different AIX OS levels. Make sure you are running the highest levels for AIX 4.1 and 4.2 with relevant device PTF filesets installed from IBM's fixdist servers.
The IBM EtherJet cards are reported as not compatible with AIX ThinkPads.
AIX Credit Card Adapter Ethernet II cards should still be available
from IBM (at least they are in Canada).
Snoop around usenet a bit and you should be able to find some used cards of both types for sale relatively inexpensively.
Also note that unlike modem cards which I reported on before, it appears
that no non-IBM ethernet cards will
work on the AIX ThinkPads, even with ODM hacking. Cards tested include Xircom, 3Com and Megahertz.
"Combo cards" with both LAN and modem functions almost certainly will not work. Anectdotal reports and any additional info is welcome. Make sure you include your AIX OS level and as many card partnums as you can find!