The choices are more limited for pistol, because the pistol is inherently less stable than the rifle, so WEIGHT and SIZE become immediate considerations, IMO. Rifle scopes on a pistol tend to get real "tippy" real fast, because the bigger, longer and heavier the scope, the more top-heavy the airgun becomes.
For Hunter Pistol, where clicking is not allowed, a smaller (compact) and lighter scope is a good choice and slightly easier to find. But for "Limited" Pistol, you may want the smallest rifle scope you can find with the most robust clicking elevation adjustment.
So, for Hunter Pistol, the UTG 3-12x 44mm compact mentioned by Paul is a good choice for many. Not sure you can find it for his suggested $120 with the etched glass reticle (probably the lesser model at that price), but you will find the etched-glass version regularly for around $160. You want ONLY the etched-glass version. Link here:http://www.opticsplanet.com/leapers-accushot-3-12x44mm-illumination-enhancing-mil-dot-rifle-s.html
I use the UTG myself for its Large Objective lens, compact size, relatively light weight for a 30mm tube, Side Focus and clear, etched-glass (mildot) reticle. But I shoot Hunter. Someone shooting Limited Class will have to comment on the clickability of this scope. I have my reservations on this scope for repeated clicking as used for Limited, but I may be completely wrong...
It's a bit older now, but this thread about pistol scopes was pertinent at the time, and I don't think the available options have changed much sinhttp://www.network54.com/Forum/451309/message/1387725870