Hello,
I needed the same thing several months ago for my xs Ultimate Sporter, and found info. a little hard to come by. Anyway, after researching a bit, I ordered two items which have worked perfectly. From Optics Planet: 1) Anschutz Rail Kit, Black, BT28. 2) From the auction site (and there are lots of sellers), Tactical aluminum 6.5-9 inch pan tilt adjustable bi-pod. The latter is a Chinese made version--rip-off--of the Atlas, and surprisingly well made and sturdy. Shipping was delayed, but I was not disappointed in the product. I believe Rapid Air Weapons also makes a rail adapter that will fit the Ultimate Sporter. I only went with O Planet's b/c RAW was sold out at the time.