Martin Ritchie makes Marines in parade formation, but he may also have made combat sets. He only sells through mail order, as far as I know. His address is :
PO Box 323
Gilbert PA 18331
If you co back to the classic toy soldier makers, I think some Timpo GIs could stand in for WWII Marines. King-White/Reeves may also have had a couple in their catalog. They, too, made GIs which could stand in for Marines, probably with some repainting.
Among the newer makers, I also think of 21st Century Toys/Blue Box, they might have had some in their catalogs.
Prost!
Brad