I have a couple of packs -- one is an Aktiv, one is an ABC, and a couple of practice packs made from surplus cells. The Aktiv came from
www.intrepidrc.com. The ABC came from a local hobby shop. The surplus cells came from an electronics surplus place near me. They are ALL nicads -- I have no idea if nimh are available. Given that the WWS, ABC DTM SP, Aktiv, etc all use this size cell, I'm gonna go out on a limb and guess they shouldn't be that hard to source. Worst case, I'm sure you can order packs from Horizon. I would drop intrepidrc a mail and see what he can tell you.
The other option you have are the flat brick batteries from someone like Blind Batteries. I think you could assemble a pack without much difficulty. They are nimh, decent mah, and not too expensive. I'm not sure they still sell them, however...