There are a number of common LiIon sizes with a short cable and a connector.

They allow you to find multiple manufacturers with similar products.

I'm not sure about availability of encased batteries similar to what is used in cameras and phones. They tend to have custom form factors, making it hard for the little guy to find good sources of them unless using a model that is very common (such as for some Canon cameras) where there may be lots of companies making non-brand alternatives. Of course, it's a bit tough with the contacts since you can't buy the receptacle. But for some batteries, it's possible to solder cables to the connectors on the battery.

Just a link about playing with LiIon batteries - I'm not sure how many will visit this forum thread but I feel a bit of info is needed to give people ideas that may hurt them.
http://www.ladyada.net/learn/lipoly/

And this is good information for people who think about charging Li-Ion batteries:
http://batteryuniversity.com/learn/art..._batteries