I forget what the policy is for posting links, so if this link isn't kosher feel free to remove it.
Like was said above, Conraua alleni are frequently imported and called "Goliath Frogs" because they're in the same genus as true "Goliaths". I have seen them for sale before in the U.S., but it has been quite awhile.
A quick search yielded this link.
Allen's Goliath Frog (Conraua alleni)
Good luck!
~Alex
Also, if you're looking for a fun read on real Goliath Frogs, Peter Brazaitis has some cool stories in his book about going to capture them in Africa for an exhibit at the Bronx Zoo!