There are online retailers, but lets say you can't find one. IF they are the type with the shrink wrapped cells and a wire, you can find out what kind of cells they are and order those ones. Then you need to find someone who knows how to use a soldering iron.
If it's the square kind with the little contacts, it's more difficult. My brother mentioned you can often cut a laptop battery open and do the same kind of thing to replace those cells.
I recently bought 2 pairs of Cobra Walkie Talkies from Circuit City's clearance sale. They take a small, special rechargeable battery. I went onto ebay and got a few extras. With ebay, you may not always find the battery for your model gadget, in my case I had to find what other models my battery would fit. Because it's not outdated I was able to search for accessories to my model radio, then go to that item and it showed the other model numbers it fit.
http://www.batteries.com
I don't think I've tried this place.
Speaking of recharging batteries, I have picked up a few AA and AAA lately plus a wall charger and a cigarette lighter charger. I figure I can get a wire and female cig. lighter and connect it to the battery terminals on my solar fence charger and use that to charge my AA and AAA if the need arises.