This is a link to a care sheet that I found:
I would say to go to your local library and print out about 30-40 copies of it and place the copies next to the betta vases, in a pile at the cash register and maybe have the cashier give one out to someone who is buying a betta, hand them out yourself if you have the time or get someone else, or put them in a pile on the counter next to them. Just basically somewhere very visible.
Also if it was me in this situation I would either write a letter, go to the manager or if one of the bosses were there, I'd see if I can talk to them about some kind of education for water changes feelings, ect. Or even the whole idea in general.