Desktop

Kingston makes a network card with a coax connector on it. Model KNE-PC2T/BT.
Here is the url.

http://www.kingston.com/networking/adapters/knepc2.asp

Sure. I found a dealer that sells two brands of PCMCIA that connect to 10-Base2 (thinnet):

http://www.amtron.com/pccard/ethernet.htm

Yes these cads do exist and you can locate at Network Accessories Computer network ethernet cards,PCMCIA Ethernet Card,ActionTec PCMCIA Ethernet Card,Networking-Hardware from Alltech Electronics

3Com makes a combo card. The Etherlink III Lan PC Card for 10BaseT and Coaxial cable. Model # 3C589C.

Some 3Com PCMCIA NICs support 10BaseT as well as 10Base2 networks. The best ones I have worked with are the 3Com 589x series. 589B/589C/589D/589E, etc.

Drivers are easily found and used.
You may have to force the NIC at the hardware level using the install/setup commands at the DOS level...since I have found that NT sometimes defaults to 10BaseT.