I started with the 3com US site and was redirected to USRobotics site as they handle all Modem Only PCMCIA cards. they had various files for download so here is the link to the drivers page where you can choose the one you want. Mac OS Drivers as well. Enjoy
The following files are found inside the driver download file.
|README.TXT||181 bytes||28 May 1998|
|COMPRESS.3C||928.2 KB||28 May 1998|
|INSTALL.BAT||133 bytes||28 May 1998|