I do not know of any place that specializes in linux and audio cards.
I would buy from a place that sells music gear:
same day music
unfortunately you won't get any linux support from these places. You have to search out the community for that.
From my own searchings, the M-audio Delta series PCI cards seem to be well supported, and you can find them at decent prices. You can get an audiophile 2496 for about $100 which has two channels analog I/O, plus 16 channels midi I/O, and spdif (digital) I/O. I've learned a lot from the ardour website ardour.org
welcome to linux. It's a great OS