I saw someone on ebay selling an electric mountain bike with the (Li-Ion) battery mounted in the front hub. It was probably a scam as all of his feedback was negative, so I don't want to contact him or anything...

But does anyone know where to buy those hub mounted batteries? I think I'd rather have my battery in the hub (assuming my motor is in the other hub). Probably help with the balance, and make the bike look more like a regular bike too.

Small folding bike, hub mounted motor, and hub mounted Li-Ion battery. That sounds about right to me.