First thing is call your local gas supplier (welding supplies) and ask them which tanks they will take, which ones they won't (if any) and if they care about the hydro date.
In my area, Central welding will take any tank not on rent (where they own them, and just rent them out). They will take those too if you can prove you are the renter, but if you're not the renter, and you come in saying you just bought it off craigslist, they will take it assuming it was stolen.
They will also accept out-of-date tanks... they give you a nice new one, and send yours in for re-hydro at no charge.
In my area you can get a 5 lb tank off craigslist for ~$30, or a 10 lb for about $50.
fills are $13 for 5 lbs, and $18 for 10 lbs.
I originally bought a 5 lb for $25, and took it in and upgraded to a 10 lb (full) for $35, so that means I got a full 10 lb for $60. It was a brand new bottle.
If I was to just walk in and buy a full 10 lb, it would have been $100.
Craigslist is your friend, but they go quick. Good luck.