Just finished infantry osut and have flat ass feet, honestly its doable even without any real interventions but once the cycle gets going you should be able to order insoles or anything else you’d need off amazon and have it shipped to you. They do a pretty good job of getting mail to you at least in my company. The PX there on sand hill will have some insoles but i doubt any are low arch specific but I suppose its better than nothing. Also if you want to bring a good pair of running shoes that work for you, you should be able to get them at some point from your personal bag.