go to a gun show. i don't think they ever check ID there. that being said, .357sig can be used in pistols, and that's all that matters for it to be called pistol ammunition. owning a vector MP5 clone won't change the fact that you are trying to buy pistol ammunition. i agree that its a stupid law, especially when guys are denied at walmart all the time for buying .22lr because it can also be used in pistols. if you get a 21yo friend to buy it for you, do not talk or express interest or an opinion on the ammo. hell, don't even walk in with him.
i THINK aimsurplus only wants you to fax a drivers license to them and they will waive the adult signature upon delivery, but i'm not 100% sure. but i'm sure they don't carry .357sig.