If you get a Pelican case, any Pelican dealer can get you replacement foam. I use the 1700 cases for my M4's and PTS ACR. I just swap out the foam for the different guns.
A word of caution about traveling with gun cases. Make sure your case is 62 linear inches or less (length x width x height.) Otherwise it is in the oversize category and is subject to an additional baggage fee of $100 USD to $200 USD, for each bag, each way on the flight. For example, the Pelican 1700 is under 62 inches so no fee. The Pelican 1720 is over 62 inches and will cost you at least $200 USD on a round trip flight.
Pelican has a nice chart for you to use:
http://www.pelican.com/airline_allowance.php