Dan, Batteries Plus can make you a battery but it's probably not worth it. I took mine apart trying to figure out how an alleged vacuum cleaner could make so much noise and not suck anything.
The Eureka battery looks like 3 size C rechargeable batteries stuck together in shrink film tubing but it's made in France so they could be an odd size.
We are currently using a Shark 15.6 volt cordless. It is great. Sucks good and the beater attachment actually works. The problem with it is that it doesn't fit in the cabinet so I store it in another area. The vacuum itself will fit but not the charger bracket it plugs into.
Check out Home Depot. They've got some really nice cordless vacs.