Destackers are only sold through a couple distributors, and they are not that easy for consumers to get since they're primarily used in commercial applications. Your best bet (I think) is to go to a DirecTV installer in the area and try to get them through them. Holland Electronics is the largest distributor AFAIK.

What a destacker does
The signal from the dish runs at a specific frequency range on the coax cable. However, the cable is capable of carying much more data than that range from the dish. So, in order to feed multiple customers off one cable, they do what's called stacking the signal. This is where they take one signal and assign it to a different frequency range in order to fit two or more signals on a single cable. Your A50 is able to natively use the unconventional stacked signal. Every other DirecTV receiver cannot. The destacker is an external device that takes the stacked signal and re-assignes it to the proper frequency range on the coax in your appartment.

This info is coming out of my head from what I remeber. It may not be perfectly acurate. I hope I described it clearly enough (probably not).