sports bras 100%. get a size down if needed but no smaller than that !!! ❤️

I had a pretty small chest so this definitely won’t work for everyone but I used to layer 2 of those not-quite sports bras (idk it was just a different material) and put an undershirt over top to smooth it out and then whatever shirt/etc over that I did this while in Disney World in August and was fine

Someone with bigger chest here - sports bras were my first binders. Looking for Nike brand at the thrift store worked well, or any brand that has that less-porus stretchy material. Be careful with double-layering. I've gotten stuck many times. Also highly recommend button ups with heavier material! Corduroy and flannel are great. Not super summer friendly, but having that on while inside is a great confidence booster!