I have a supply of Astia from Freestyle, but I think they've sold it out, and it's becoming more of a special order item.

That said, you should be able to find a few types of E-6 and color neg film from B&H. Adorama has more on their website than they really have in stock. Also check Badger Graphic.

As to whether you choose to start with slide or neg film, see what the instructor advises before you stock up. 8x10" color film ain't cheap! If you shoot positive film and want to print conventionally, your only options right now are Ilfochrome or internegs (I'd go with Ilfochrome). If you shoot negative film, then there are a number of printing options that are less expensive than Ilfochrome.