how small of a monitor are you looking for? size limit? im no expert, but here is a start:
http://www.kingsnake.com/monitorfaq/species.htm
http://www.reptilemall.com/caresheets/varanids.html
those links provide very gen info on monitor species' sizes and cage req'ments. while there are smaller or "toy" species that are legal to purchase, they are VERY expensive to initially aquire. but you probly already know this. though even with smaller species, the animal still requires a very large cage.
Ackies, or Ridge-tailed Monitor, Varanus acanthurus are fairly small, 2 feet approx.
http://www.kingsnake.com/monitorfaq/species.htm#7.1
if you dont find a monitor species as small as you like, Tegus are very "monitor-like" in some ways, and more managable in size (of the actual animal). they do still require ample cage space. Tegus also usually have a better temperament, but just a generalization.
hope the links are a helpful start. something else, once you find a suitalbe species, do research on breeders of such exotic species, before you spend alot of money. the small species you are looking for are sometimes hundreds of dollars for a hatchling. good luck!
~Kelsaurian