The best place to go would be a reseller who sells adtran products. You can also buy it from adtran directly. (www.adtran.com). I think the cost of the unit is a 1000 dollars. I had one in my test lab, the unit is modular, you can add modules such as T1 pri, E1 pri etc. The unit also has a 4 port bri switch which helps in creating an ISDN cloud for lab testing. It also provides a t1 interface, which helps in creating test beds for testing CSU/DSUs. This is a great product and a must for all time CCIE lab students.
Sankar Nair
UC Solutions Architect
Pacific Northwest | CDW
CCIE Collaboration #17135 Emeritus