What's the best place to buy an assortment of imported beers?

Hello Natilady, Dutche's in Hyde Park has quite a large selection of imported and craft beers.
http://www.urbanspoon.com/r/32/1584637/restaurant/Hyde-Park-Mount-Lookout/Dutchs-Bar-Bottle-Shop-Cincinnati

As SuzM said, Jungle Jim's is great. I recommend it as well, especially for anything imported. If you live too far, though, or can't find what you're looking for there, definitely check out the Party Source in Bellevue, KY. They're right across the river and have a great selection of all alcohols, including beer.

Jungle Jim's in Fairfield has not only the best selection of wine in the city, but also the best selection of beer from all over the world. Jungle Jim's is a one-of-a-kind specialty grocery store (although a second location will be opening later in 2012) that features a variety of international food halls, fresh (as in live) fish, a huge produce department, fresh baked goods, and much, much more. Jungle Jim's has 1200 specialty, craft and imported beers, so you can find just about any beer you could think of at the Jungle. Jungle Jim's also has a large ice cold beer cave and even sells draft beer in kegs and mini-barrels. Some of the imported beers they feature come from places such as England, Denmark, Germany, Brazil, Mexico, Vietnam, and Japan, just to name a few.