I got mine on eBay. Did you look there? If not I'd try contacting the fellow who makes them. I have one but unfortunately can't use it because I do not have a Disk Manager cart.
I'm a newb @ TI99/4a stuff so I could be wrong about needing that.
~telengard
I'm a newbie with the TI-99 as well :-)
I suggest getting an 'Extended Basic" cart as a start.