D-I-C-K's Sporting Goods is having a one-day sale today (7/10/2019) , and their 20oz paintball tanks are on sale for $20. I think you have to purchase the tank online, but you can order online and pick up at a local store. According to their website you can get the tank filled for free the first time with the receipt from purchasing a new tank from them.

Just a heads up, I have no connection with them, just noticed the sale. Can't link to it because of the obscenity filter here, LOL!