One source for reasonably good 6V batteries is from industrial manlifts. Many of these are 24V systems using four 6V batteries run in series. Usually when one battery shows problems they'll replace all four. The remaining three may or may not have any problems. There may be a small disposal fee (like $2 a battery) but other than that they're usually free for the taking.
If you know anyone that works in a warehouse or distribution center you might have them keep an eye out for them. I got my batteries from work that way and out of eight only one had problems. The others were just a little low on water. Some of the times the problems are with their chargers instead of the batteries.
You might also check local material handling/forklift service and rental companies or of course, the battery stores themselves. They'll often be the ones servicing equipment or selling the batteries and will collect the old ones.
Hope this helps,
-Blue
