Well, you didn't say which area in the US you're moving to.
However, the cheapest way to get it done (once you and the car are in the US) is:
1) to look on www.craigslist.org
for taillights (it's nation-wide very well known site for used stuff; puts you in the direct contact with sellers)
2) go to a junk yard
BUT, all that said, the best way is to just leave your car as is, because the amber signals are perfectly legal in the U.S. In fact, half of the cars have them in amber (all imports 100%).