U.S. copyright law states that the one who took the photograph is given copyright protection from the moment the photo is created. This includes digital images. Legally, if you want photos to use, either the photographer or one of his/her agents (like the wholesaler) must allow you to use the photo(s), or you must take the photo(s) yourself.
I am currently selling preowned clothing and I understand where you are coming from. Finding a mannequin where the garment looks nice can be a challenge. There are different ways to display clothing other than on a mannequin and still have them look nice. Take a look at what some of the retailers do online for ideas.