Standard crap's pretty much available from the markets in town. Prices vary depending on the district, so it's good to shop around a bit. The magically active plants come in from Utgard or the Alfheim folks. They won't sell to you if you don't have a license, which takes about two weeks of red-tape over at Utgard and a test to get through. If you know a lick about alchemy and how not to blow yourself up, you'll be fine on the test.