I do not know of any place that specializes in linux and audio cards.
I would buy from a place that sells music gear:
musicians friend
music 123
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