Alan wears vintage demi tortoise BluBlockers sunglasses in the movie. You can buy the original ones, or buy these similar looking. The original one is a little expensive, so I recommend you to buy the similar one online which has the same fashion style and cheaper price. Here is the link:
Yes of course, as a matter of fact, I have watched that movie, quite impressive in any way. So, the type of the sunglasses Alan wears in the movie is called aviator sunglasses, which is invented by the american glasses producer in the 1930's. You can get the sunglasses at all large optical centers or shopping malls, here my recommendation for you is Bergdof Goodman, which has a lot of options for you.
Yes, I have seen the movie of hangover. It is the 'Demi Tortoise Nylon BluBlocker' sunglasses that alan wears in the movie of hangover. If you want to get such type of sunglasses, you'd better go to the online stores to have a search and buy one. And the price online is relatively cheap.