I have a Comtol Rocketport 8Si. Works great so far and has Linux support (can actually share its 8 ports b/w Win and linux machines). I got mine (new) on eBay, and I see there are more up for sale - try here.

(edit: I'm not using mine in-car. It is a bit big. FWIW, they also make a 4-port model that might be smaller. Another company I have liked is lantronix; they have a 4-port MSS-4 that is popular enough that you might find one on eBay sometime. Linux drivers? Come to think of it, not sure why you'd need them to make this work...)