Years ago, I tried to get a monitor at Circuit City because the ones they have hooked up to their video cameras seem about the perfect size to me. I was told that they don;t sell that kind, however to consumers.
I currently use a 5-inch black and white TV I got at Albertsons (a grocery store or a drugstore may have these as well) at Christmastime. It was under $20 and as long as it has an input for video game consoles, you can hook it right up to a camera.
The drawbacks: The edges aren't specific, so you have to be prepared for some things that seem cut off on the monitor being visible in the uncropped frame. Also, I've been noticing as the TVs get older, they've developed a "buzzing." They're good for practice and lipsynching to records -- anything that doesn't require you to be mic'd.
Someone on here suggested using one of those portable LCD DVD players. They cost about $100, have inputs, are in color and come with rechargeable batteries. I may be investing in a couple of those soon.
Steve