I don't have petcos here but I'd say that every animal/fish/living being is unique on its own way, so I guess it's pretty much luck in what you will find, of course the chance of getting some "rare colours" are.. well, rare, but nothing impossible, since lots of people in the US buy thai fish to breed their own, so it's not alarming to find some of those fry getting into market/fish stores. =]
The other month I was in a petco and I saw a red HM with some lovely silver scales and a white rim aorund his fins. He was amazing, but I had no money at the time for him.
Ask your petco when they get their betta shipments, and visit on that day to see the best of their stock. People who don't know anything about bettas still know the difference between drab and flashy, and are likely to snatch the good looking fish up before you have a chance to.
Also just wanted to add, that some fish who look drab color up lovely when you get them home. The fish my my avatar, Volker-- he was mostly brown looking when I saw him in his petco cup. However I took a closer look at him and saw little hints of his blue scales-- and in the light I saw that he had yellow fins. He looks lovely now.
The pic in my avatar is my HalfMoon from Petco. He is blue and green in real life. The picture makes him look all green. ScoobyBlue is super active and has been great so far. He has alot of personality!