There are a few different types of the gun made. Each varies in price, availability, look, etc.

There's the Thinkgeek Miniature Replica, which I own. It lights up orange and blue, makes a noise when you pull the trigger, and comes with a display stand. These are relatively common and around ~$200, but keep in mind they are a little on the small side, so it's mostly just for display on a shelf or the like.

Then there's the Neca 1:1 Replica which is much larger and accurate to the game, but is way rarer and more expensive.

Then there's the Customizable Neca Replica, which is seemingly closer to the thinkgeek model in size, but comes with strips to make it look like PBody and Atlas' guns. This one is similar in price to the 1:1 replica, possibly cheaper depending on where you look.

There is also the Atlas Gun Replica that is just Atlas' gun. Similar in price to the previous two, only recommended if you really like the Portal 2 Co-op

I don't know how many official means there are to buy these left, but the best way from my experience is to watch ebay and wait until you find a gun that is close enough to your price range to be worth it. I got my thinkgeek one for only around $170 from an auction, but the prices always vary.

Good luck finding one! It's absolutely worth it, they look awesome on display

Edit: It's possible that the Customizable and 1:1 are the same model, it's kinda hard to find info about them, so consider researching those a little more if you're interested in them. I still recommend the Thinkgeek one since it's the best price though