I have the same problem. While I will wear a shell underneath to make a neckline a little higher or for sleeves, I don’t buy dresses that have cutouts, dips in the back, v-necks or very low necklines in front, etc. It makes for an extremely difficult time.
What I try to do is find a dress that’s workable- meaning, it has enough material in most places, but needs to be lengthened a little. Then I’ll buy material and add to the bottom. It’s expensive, but with my standards of tznius and also my desire to look nice (not trendy, but put together and attractive), it seems to me to be the only way.
Lord and Taylor sometimes has good sales on dresses that are “less problematic.” Nordstrom is very popular among frum people, but I always wonder why, since most of their dresses have weird cutouts and are also extremely expensive. Some of the frum stores can have dresses without the cutouts that “only” need a hem down.