
I found some ideas on etsy, a tank that pulls down over a built in top. But I decided to come up with something on my own, hopefully for a bit less $$. I lucked into a big sale at Old Navy, where I ordered a couple of knit blouses for $3 each and some of their "first layer" tank tops. And then picked up some actual crop top tees at Target for cheap.

I turned the knit blouses into crop tops by cutting off about 3 inches on the bottom, hemming the new bottom of the shirt, and then running 1/4 inch elastic inside the hem so the bottom of the shirt would stay under my boobs until I pull it up.
The t-shirts I just wear under the tank top as they are.

To nurse, I pull down the scoop neck of the tank top and pull up the bottom of the blouse. After the kiddo is latched I adjust the blouse for coverage.