I would just suggest being a bit wary of pawn shops - first of all, the proprietor is likely to have no idea on how to correctly price a bicycle. This can work against you, but it can also work for you. More likely against you. Second, they're not likely to have much of anything. Finally, there is the disturbing possibility that they could be selling stolen goods. It wasn't a pawn shop, but I was greatly creepified by a few guys I saw selling a bunch of bikes out of a van during a trip to NYC. I had my doubts about the legitimacy of their business.

A local bike shop might sell or might be able to point you in the direction of a shop that sells used bikes. Look at classifieds in the newspaper and on Craigslist.

As for what you should look for, well, it's pretty much the same as any other bike you might be looking for, with a few special caveats. First, something that has or can take a rear rack to carry stuff on. Second, a bike that can take what you throw at it. It doesn't need to be bombproof, but if you're going to riding on beautifully paved streets in sunny weather year-round, or on the streets of Hell itself in freezing, blustering blizzards, shop accordingly. Generally, the more variable and rough the conditions you'll be riding in, the more you'll want to err toward "tough/indestructible/comfy" as characteristics and the less toward "fast/light/aggressive." I've personally found that my touring bike makes a nice middle ground - pretty tough and pretty comfy (I've taken this bike on gentle off-road trails in the woods, and it does fine), but also reasonably speedy on smooth roads, and not a hundred pounds, although it is considerably heavier than most would consider acceptable for a road bike. Lots of folks tend to agree with this assessment, and so you'll find a lot of commuters on touring bikes, and on various flavors of hybrids. In fact, I'm fairly certain that hybrids came out narrowly ahead in a recent poll taken in this forum.

The problem with giving advice on what to look for is that there are nearly as many commuters bikes as there are bike commuters! There are folks that commute on hi-tech carbon fiber road bikes, and folks that commute on bomb-proof, 15 year-old steel MTBs, and plenty in between. There appears to be only two points on which there appears to be some degree of consensus, and even these are not universals by a long shot:
1. Full fenders. You're just gonna need 'em. Look for a bike that either has them, or that you can install them on.
2. Some way to carry your stuff on the bike - either it has a rack, or braze-ons so you can install a rack.

Everything else - gears, frame material, bike style, tire size, whatever, is pretty much according to your taste. There are a lot of people here that like gears, steel-framed hybrids or touring or cyclocross bikes with medium-width tires (28-32mm), but there are also a lot of folks riding single speed or fixed-gear bikes, aluminum or carbon fiber-framed racing bikes and using skinny little tires, even on nasty roads. The best advice I can give is to try and determine, to the best of your ability, what your needs are, and then try to meet them. If you tell us what your specific needs and requirements are, we might be able to help you out with more specific advice, but until then, this is all I can think of. Good luck!