There are techniques for de-sulfating a battery that has been left to sit uncharged for too long - but there is a limit to the amount of lead sulfite that can be pulsed off the plates and put back into solution. In general, if there is a bulge under the positive terminal, or a very noticable bulge in the side of the case - you're better off buying a new battery. The 6TL battery ("large, square" one) is available from most large battery distributors, such as Exide, etc. Price will be around $100 for a new one - depending on where you live. I have never bought one in MO. The 6TMF or 6TLMF are newer, maintanance-free replacement types for the 6TL. They have a slightly lower cold-cranking capacity than the 6TL - and I personally prefer to have the option of adding water or electrolyte. Optima Military battery Systems also makes an adapter tray to adapt an Optima battery into the 6TL profile. Great idea - but the adapter costs more than the original 6TL!
BTW - there are a number of poorly-made knockoff 6TL's that are made in China and Korea, that don't last very well. I've had 2 (sold by Sunbelt) that shorted on me, while in service. You should have heard the "Bang" as each of them went out!