I just saw two nice sets of Army stamps (one of images & one of sentiments). They were clear stamps, but I cannot remember who the manufacturer was. Ugh.
Inkadinkado has emblem stamps with the insignias for the different branches.
DRS Designs has the seals/emblems also & many sentiments to go along. (
http://drsdesigns.com/patriotic). They attend some stamp shows, so if you are not in a hurry, you can see if they may be coming your way this year.
I also saw that A Fistful of Stamps has a bunch of
army and marine stamps.
I can't vouch for the quality or customer service of the above retailers, but I hope it helps your search!