in my area, the quilt stores, including joann's (phtoot!) "hire" people to make up their samples. they supply the fabric, the patterns, and the notions, and they pay a pittance. after it's finished being displayed, the maker has the opportunity to buy it at cost, or some stores give it to the maker in exchange for the work. if a display quilt has $350. worth of fabric and batting in it, and you like the quilt, you may feel it's worth it to you. you would have to make your own deal with the store. there are no rules.
EDITED: some stores only use people they know from classes that they offer.
the idea of bringing a sample or a portfolio is a good one, especially if
you've been shopping the same store for a while and they know you.