Where to find undergarments for vintage (1980s dress)

So the dress was originally in 80s, reused in the 90s, and when I took it out of storage, it was is good condition, just a few sizes too big. Unfortunately for me the dress has no structure to it. My family's explanation of bustiers and petticoats and nylons went over my head. I see that David's Bridal has some petticoats, but how to I go about finding and getting measured for a bustier to give some structure to the dress and support for my chest in what is essentially a strapless/off-the-shoulder slip? I live in a Minneapolis suburb and my wedding will be in central Minnesota. Hoping to figure out garments before I check out vintage alterations. Thanks!