If certain rechargeables are hard to find or expensive, you might consider trying the "zap treatment".
I just bought a used Makita 12V cordless VSR drill/driver on eBay for parts. It came with two absolutely dead battery packs. These are obsolete and run something like $40.00 each, so I thought I'd give 'em a try.
I bought a "slave" flash unit from the local camera store ($20.00, runs on two AAA's) and hooked up wires and my multimeter as described in the article. I disassembled the battery packs, cut away openings in the insulating cardboard tubes, figured out the batteries' connections, and methodically zapped 'em once each, one by one.
Worked like a charm! (So far, at least.) I can't yet vouch for their longevity, but the two packs each hold far better charge than do my own original batteries. (They're next.)
One thing I did that I won't repeat: For convenience, I usually zapped the batteries by touching the electrodes to the batteries' cases. Since then, I've reflected that I may have weakened the cases and invited premature leaks. (Each zap leaves a little "spot weld" mark.) Next time, I'll do the extra work necessary to touch only the spot-welded metal connectors that hold the batteries together.
Anyway, just thought I'd share -- the technique is effective and (more importantly) very satisfying.