Look for "t-shirt bras". They are very similar to sports bras but are more like... I guess, real (?) bras. They look very much like a normal, regular bra but are usually made out of thinner material, usually do not have a clasp of any sort in the back (meaning the vast majority are pull over style) and they are designed to not be noticeable under the average t-shirt (hence the name, lol). They have no under wires either, which helps.
Really, though, and at this point I'm basically echoing what has already been said, just look for un-padded bras with no under wire. As other people have said, sports bras and bikini tops would probably help. Baggy shirts help too, which seems counter productive since baggy shirts are "girly" or "feminine", but if stealth is your goal, you may want to consider baggy shirts and a hoody as they really do help conceal the bra underneath.