The least expensive scuba tanks are used.
The least least expensive used tanks are aluminum.
Whether you dive with aluminum tanks or not should depend on where you dive, not on what the tank costs. Aluminum tanks are normally more appropriate for warm water diving than for cold water diving. You need to wear about 4 to 6 additional pounds of lead for each aluminum tank, therefore if you are already weighed down with lead from your exposure suit, then 4 to 6 lbs more may be the staw that broke the camels back.
Whether or not you buy tanks at all depends on how much you dive locally. If you dive less than once per month, then renting tanks is probably better for you financially than buying them would be.
Scuba stores normally sell off their rental scuba tanks about every year or two. That is the best time to buy used tanks. Look for a sale like that. Check each of the local stores in your area.
If you are already a regular customer of that store, you are likely to get better service and more honest opinions from them as to whether a given used tank will be good for you in particular.
Before you buy any tank, you should be able to look inside of it with an illuminating scope and see if it has any pock marks or oxidation. In steel tanks, the oxidation is brown. In aluminum tanks, the oxidation is white. All tanks oxidize, faster if they hold nitrox, even faster still if there is moisture in them.
Any abnormalities inside a tank, like a discoloration line, is a clue that something else might be wrong with a used tank. The best tanks are shiney on the inside, top to bottom, with no cracks of any kind near the threaded neck area. Aluminum tanks tend to develop cracks over time. Steel tanks tend to rust.
You can usually find one or two year old used tanks at a scuba store rental gear sale for under $100 each. Inspect the inside of the tank, then write down the serial number and have the store vis it and fill it for you. Take care of it, and the tank should last you at least 5 years. If you dive once a month, for 5 years, then you will be saving money over renting tanks each weekend. And you have the convenience of having your tanks with you to go diving whenever you want, if you keep your tanks filled all the time.
I would not recommend buying used tanks from any other source than a scuba store.