Here is the link to the supply house that has this item in stock.
http://www.hosfelt.com/Adapter%20Folder/Adapter%208.htm
This data is copied from that page, including price.
Mfg. # WP571420G
3-1/2"x 2-1/4" x 2-1/2"
Input: 120 VAC, 60 Hz
Output: 20 VAC @ 700 mA.
Class 2, plug in transformer. The enclosure is black. Output via two screw terminals.
PART # 56-526
$2.75
By the way, these same folks stock the Atari game transformer to run an ART Di/O.