I got the ones from rdana off of Ebay. These supposedly will work with roughly 80% of the Maha C9000s out there, but were also designed with the Opus BT-3400/3100.
I paid $8 delivered for them, if that.
I think that they actually stayed put in my Maha, but sometimes popped out on my Opus 3400 when I fiddled with them upon receipt. I don't have any D NiMH batteries yet, so I can't say how they work in real life.
Chris