I'm going to start by assuming that you're asking about regular DVD, not HD-DVD (as far as I can tell, only season 1 is out in HD yet).
You should be able to find it at most big media retailers. If you're lucky, you'll even be able to get a two-for deal on the two halves of season 2 (which, for some obscene reason, was split into two packages). I can't really recommend any specific retailer, as I don't know where you are, but for us Toronto-bound Canadians, Best Buy and Future Shop have been pretty good. Failing that, you might try (again, in Canada) someplace online like CD-Plus or DVD Boxoffice.
<some few minutes later, after some checking>
OK, DVD Boxoffice won't sell you season 1. And they have no deal on the two halves of season 2 (neither do CD-Plus, although they do have season 1 in stock). I bought mine at Best Buy/Future Shop, and at the time (around January this year, maybe February) they had the two halves of s2 bundled... they might not anymore.