I have a friend who rides an electric assist bike. He's a bit of a tinkerer.
Keep in mind that there is no such thing as a non replacable battery. I regularly crack "non-servicable" battery packs for my power tools and cameras and replace the cells inside. You need to be willing to solder.
Check this place:
www.batteryspace.com - they have almost everything.
My friend who has the electric assist bike bought some F cell NiMH's from there and used them to upgrade the NiCad pack that came with his bike.
I don't think any bikes are using LiIon, the discharge curve doesn't match, but if you play with them be careful, if you hook them up wrong and overcharge/overdischarge them they can explode.