Stan,
There are two types of tank depending on your budget.
Steel - heavy but robust and readily avaialable second hand.
Fibre wrapped - light but very expensive.
If you can afford it, get fibre- you'll understand why if you have to lug your tanks over any distance to the safe area.
When buying your tank make sure it's in good condition - no scrapes, dents or other obvious signs of damage.
You'll also want one thats well within its test date. If it's about to run out this should be reflected in the price.
Go for a large capacity with high pressure fill if you can as this will last longer.
If possible ask the seller if you can get the dive shop to give it a quick look at. They will tell you if there are any obvious problems.
Happy hunting,
John
