Hey
I would suggest
thebreastformstore.com. They sell some very good gaffs. You would have to learn to tuck properly to wear them and it initially takes some practise to put them on without hassle but it's worth it. They are a bit expensive but they last ages. I would get maybe 2-3 pairs initially to find your size and fav colour (although really the colour becomes obsolete as you have to wear a pair of panties over it anyway). Then try and get cotton panties on top. Unless you're exercising intensely and/or wearing leotards etc, you should only need the one pair on top. Otherwise move onto a second as well.
Gaffs are very important if you want to be tucking. Normal cotton panties (cotton is non-negotiable - you need at least some breathing down there) can't hold a tuck in place. Trust me, a half-undone tuck is incredibly painful, especially when sitting! While very occasionally a gaff can pinch, its worth it for the ability to hold everything there. You can even swim with one on
For anyone that's wondering: A gaff is a special kind of thong that is bigger and stronger at the front as well as more elastic than normal underwear designed just for tucking. If you're not into thongs, it kinda sucks cause there isn't anything else out on the market really but its a price worth paying if having a bulge really bothers you. You get used to it.
Oh and btw, gaffs are referred to as being for "men". Kinda stings but I guess the crossdressing community uses it more than us. I got past that once I actually had the gaffs and realised how effective they were.
Much love
xx