Are you sure it is 7.5V not 7.2V? Usually rechargable batteries are made in multiples of 1.2V.
Jaycar sells NiCad, NiMH and Lithium Ion batteries with little tabs for soldering them to each other. It makes creating custom battery packs easy. I would be looking that way. You will get raped if you try to buy a proper Motorola pack because it will just have the Motorola label that the Jaycar stuff wont.