I wear size XS men's undershirts and I take them in about an inch along the shoulder and side seams, then they fit me. Not so feasible an option if you don't sew, but I got tired of them always being too huge and baggy.
I also sometimes wear the largest boy's size, they're a bit small but they fit, just shorter and higher in the underarm and neck than I'd like... I use them more to smooth out binder lumps than as a regular undershirt, really.
