You can buy so called absinthe at the corner liquor store in many places in the United States (like where I live in Brooklyn, NY) but it's not going to be the real thing. Sure it'll be cheap and it will taste similar but it won't give you that special Absinthe effect that you are looking for.

To get real absinthe you have to purchase it online from Europe. In my experience the best place to purchase absinthe online is at AbsintheSupply.Net

The absinthe sold at Absinthe Supply is real Czechoslovakian absinthe! Yes it is far more expensive than the local corner store absinthe but there's a reason for that. It's something entirely different.