While I applaud your effort and goal, I wonder if you have thought about exactly what you will be asking them to do. Have you considered what is involved from a space, maintenance and risk stand point? I actually think cost is a minimal item as they could probably get the bowls I've linked to below for 50 cents each if not less.
I went by the Petsmart near my office today. They keep their bettas in 3.5" diameter cups that are about 5" in height. They had 71 bettas in stock (too many in my opinion but that's another story) and they were on 2 shelves, 4 cups deep. Each shelf was 3' long, 16" deep and there was about 6" between the shelf and the next one above it so the total space used to stock these bettas was about 4 cubic feet. If they switched to a 1/2 gallon bowl like
this one which is 6.75" wide x 4.25" deep x 7" high, they would only be able to get about 15 bowls per shelf as opposed to 36 cups and would need at least another 2 to 3 inches between shelves so it would take 5 shelves at 9" in height or about 15 cubic feet of shelf space. That's 4 times more space and they don't make their money off selling bettas. They make it off selling the tanks, bowls, filters, etc. so stock space used for fish isn't really profitable.
The bowls would also need lids for a couple reasons. First, bettas are jumpers and without a lid, you would end up with dead fish on the floor and shelf. Second, no lid would mean everyone that looked through the shelf to find the betta they want would slosh water all over the shelf, the floor and themselves. That could create a injury risk if someone steps in it and falls.
Next there is the risk of someone dropping a heavy bowl (1/2 gallon filled with water is about 4 lb allowing for it not being full). There would be flying glass or plastic and water which could injure someone or damage other merchandise or get on people that just happen to be nearby. That is a risk that the don't really have now with a cup that only contains a tiny amount of water.
Once again, I commend you for trying to change things for the better but bettas have been sold that way for years and until some sort of laws are passed that states that the cups are inhumane (most people have the opinion that they're just fish) then the fish breeders and fish stores aren't going to change the way the sell them. If PETA (and I am definitely not a fan of theirs) hasn't been able to force change, a few names on a petition won't matter either.
Edit: Just a side note, FBTgirl, you and I have the same birthday on June 4 although mine was many years before yours.