Huh? You can buy it from pretty much anywhere that sells games. If your local store doesn't have it on the shelves they can easily order it through their distributor. Yes, the game is still here, still supported, and still receiving new releases. Terror in Venice, the latest expansion just came out and the next expansion which is a Syndicate faction box was recently announced.
Any individual expansions that go out of stock get scheduled for reprints, it's not generally an issue. No expansion for any LCG has ever gone permanently out of print and there's enough to keep you busy until the one you want gets new copies made.
No reason to expect a new edition, what would it be for?
There's a great online deck builder here: http://www.cardgamed...eckbuilder.html
If you set the filters to just show the Hastur faction you can browse through and look for cards that interest you. Most people start with the Core set and then get Secrets of Arkham or possible Terror in Venice for a first expansion. ALL expansions are legal, that's one of the advantages of an LCG over a CCG - none of them are ever cycled out or expired. Personally, I think the last few boxed expansions have had some pretty good cards for Hastur - you may also want to look at some of the Lunatic characters but it can vary depending on the type of deck you want to play.
I don't know when it will be, but when the faction box for Hastur comes out you'll definitely want that. We know Syndicate is next in approximately 3 1/2 months, and so far they've been alternating human/mythos so if that pattern keeps the next one will be either Hastur, Shub, or Cthulhu.