I think you have to decide what matters most to you in the monitor.

Do you want:

The highest resolution possible?

Easy portability?

A mountable screen?

An articulated monitor arm?

Battery/DC/AC power?

HDMI/Composite input?

Touchscreen functionality?

On-board speakers/sound?

An inexpensive solution?

A warranty?

The answers to these questions will help you to determine where and how to search for your monitor.

Some options:

Portable DVD players/Second DVD player screens/Portable TVs

Some portable DVD players, car rear view mirror camera screens, and portable TV screens have composite inputs. They are made by a large variety of manufacturers. They often have built in speakers and sound inputs. They may or may not come with car/AC adapters. They may have included batteries. The resolution is usually much lower than comparable computer monitors. Verify that the screen has composite input, and then verify if the connectors are on the device, or if the composite A/V input requires some sort of "AV-in" conversion cable. Re-purposing these screens for your Raspberry Pi may save you some money and still fit your needs.

Professional photography/videography screens

Some monitors have been created for use in digital photography and videography, such as those made by Lilliput. They usually have an internal/external battery and often have HDMI input, and have the option of sun shades to deflect sunlight, as well as mounts (usually for being mounted to a camera or tripod. The resolution and brightness may be the best you can find. However, the price is quite a bit higher than some of the other monitors available in this size.

Car PC / Carputer monitors

There are also a fair number of "carputer" screens, some of which have DVI/HDMI input and even touchscreen functionality. The cost for these screens is higher, and the touchscreen may or may not be easy to set up in Linux, and they may only accept DC power natively (due to being designed for use in cars) but it may provide you with a more complete solution when all is said and done.

Standalone monitors (USB)

The next set of monitors to look into would be the standalone USB monitors, such as those made by MIMO and Lilliput. Upon first glance, they seem like the best option, because they only require USB input, but display drivers may be hard to come by, and the Raspberry Pi may not be powerful enough to feed the screen the power it needs.

Small computer monitors/touchscreens

Some small computer monitors or touchscreens also exist on the market, mainly by Mimo and Lilliput again. Some of these monitors have touchscreen functionality. These are differentiated from professional photographer screens by their lack of battery power. Driver support for touchscreens may work, or may not. Tread carefully. These monitors tend to be on the more expensive side, in general.

Conclusion

If price didn't matter to me, I probably would have purchased one of the Lilliput monitors, but price does matter to me, so I didn't. I ended up deciding on a DVD "second screen" that had an included mount and AC adapter on eBay. Sound inputs are included, and it has AC power. I don't plan on my usage requiring anything too fancy in the resolution department, and I was looking for the most cost-effective way to get my Raspberry Pi running. Getting it from ebay meant that I paid a pittance ($20), but I had to lurk around a lot of different auctions to find it at that price, since they're usually closer to $50. This also means that I don't have a warranty and I'm purchasing a product more or less sight unseen, so I may not have the image quality that I was originally hoping for. In my case, it worked fine when I hooked it up, as evidenced below:

It all depends on what you value most in your screen. Your mileage may vary.