Absinthe may be legally sold in the US if it contains less than 10 parts per million of thujone and is approved by the United States TTB. This is the same as 10 mg/kg, which, according to Wikipedia, is the legal limit in the European Union of thujone in spirits above 25% ABV. According to the website Buy Absinthe Alcohol, they sell only absinthe with limits of "35mg/lt for bitters and 10mg/lt for spirits." One liter of water weights 1kg, so the absinthes (not the bitters) appear to come under the safe thujone levels in America also. Many of the absinthes on the website list their exact thujone content, which is a nice double-check.
The products appear to ship from overseas (I'm not sure where) and are marked as "not for human consumption," as they are not (yet) approved by the US TTB. Shipping fees are a little less than $30 to North America. They generally stick to selling brands from France and Switzerland, which are better regarded than Czech Republic brands. And though they say they avoid dyes and color additives, a few of the bottles' bright blue or green colors would leave me to believe that's not a hard and fast rule. Some of the brands on the website are Lemercier, Versinthe, Absente, Rodniks, and Pere Kermmans. They also sell absinthe spoons, glasses, and hard-to-find absinthe fountains, though last time I checked they were out of stock of the fountains.