The laws governing what constitutes wrongful termination vary from state to state, so you should do some research on the matter before taking action. The doctrine of employment at will is difficult to overcome in the US, but some states have enacted exceptions to it.
As with most other types of cases, the first resource to tap when searching for a lawyer to litigate your wrongful termination case is the people you know: your friends, family, co-workers, and neighbors. They should be able to give you a few leads, not to mention candid assessments of people they've worked with.