Ramirez,
The most comprehensive and best deal for your wallet are of course Old Glory. Not the best sculpts, but they are correct in their uniforms. They even have US troops including sailors for the Vera Cruz landing.
Foundry has some that will work, like the irregulars from the Camerone line, but for the most part are Maximillian are too early. They could work for early part of the war Federal troops, but for the most part, they switched to a more "modern" look very quickly.
Gringos has a nice growing line for the period.