Once I get out of my apartment and into my house I will be getting a trio of Ackies. Right now they are the go-to. You can find them for $300 or so (higher for bright color morphs). IMO they are probably the best all around smaller monitor to keep.
Other smaller monitors:
Timors - Bigger than ackies, largely arboreal, and difficult to handle. More common. CB are available
Trees - Same/slightly smaller than Ackies. Completely arboreal. Some are downright nasty. Quite colorful though. Bought the same price as ackies.
Storri - Smallest monitor I know of being somewhat available. Back when i kept monitors they could be had for $500. Now, forget it...$1000 minimum. Very rare and hard to find. Get aggressive as they age. However...most are under 12" !!