If you're up to the task of caring for a sick betta, the big chain pet stores have policies where they can't sell sick animals so you might be able to persuade them to let you adopt, which means the store gets no profit and you get a fish. (This is what I think of when I hear of 'rescuing' from stores- otherwise yes, you're just perpetuating the system) However, sick bettas can be pretty expensive (and delicate!) depending on what is wrong with them so if you're on such a strict budget, that's probably not a good idea.
If you only have five dollars to spend, and you absolutely don't want to contribute to mill breeding, I would suggest waiting & saving until you have enough to either buy medicines so you can rescue, or pay for a fish online. I know there are several breeders on this site, perhaps one would be willing to let go of a pet quality fish for the price of shipping if you asked very nicely. A fish from AB has the same chance of arriving DOA as one from here, maybe even more so since they're often flown overseas not just across state lines. Craigslist removes the shipping factor, but in my opinion is a bigger gamble since you have no real idea how healthy the fish is, it's age, etc.
If you're absolutely desperate to get a cheap fish immediately, your only real option is to buy from a store. I don't know anyone who hasn't at one time or another (myself included!), and one fish sale isn't going to change a store's decision to sell live animals.
Do you have all the rest of the supplies you need? Tank, heater, filter, conditioner, etc?
10g divided; Claude VT & Marie VT | 5g; Gabriel DeT(?) | Mom's 2.5g; Hank DeT