kat', sooooo happy to read this. You want an M tank or an H tank (plus you want to keep your smaller tank for portability). Those are the big ones. The big tanks are heavy, so you might also want a stand, or particularly a rolling stand if you have to move them. Now that you know more about O2 working for you, you can consider using welding O2, too, if your supplier is not cooperative. Since they make more money when they provide you a larger tank (or tanks), and have less hassle from having to constantly replace the smaller tanks, you'd think they'd be interested.
If you get a bigger tank, or any kind of welding tank, you'll need a different type of regulator. An O2 supplier will know this, but you'll need to know it if you go the welding route or if you decide to buy a higher-lpm regulator than the one the O2 supplier would give you. Keep us informed.