Informal wedding dresses do not need to lack the elegance or the beauty of a more traditional wedding dress. An informal dress is usually a bit simpler in its style and detailing (maybe less lace, beading and glitter overall). The great thing about going with more of a casual wedding dress is that you can wear it again and again. So it is definitely a more reusable alternative to go with if that is what you are looking for. It also gives you more options as to where to find a dress (any shop with dresses and woman's wear will probably offer something).
Informal or casual wedding attire does not necessarily have to be a dress either, it can be a two piece suit or something entirely different. This goes back to the point of being comfortable and feeling comfortable. You don't want to feel like you are getting married in somebody else's cloths or in a dress you know you would never pick out yourself, just because you think you 'have' to.
When going for more of a casual look for your dress consider a few things such as if you have bridesmaids, do you want to color coordinate with them? That may narrow down options of dresses and colors if you want to find something of a similar style, but not the same, as yours. So keep the big picture in mind even when shopping for casual wedding dresses. If you don't have to worry about color coordinating your wedding attire with anyone else then no problem!
Make sure that your invitation specified if the attire at your wedding is 'casual'. Dressing casual usually loosens people up a bit and they feel comfortable. So on the flip side, if someone comes dressed to the teeth to a 'casual' attire wedding, by mistake, it may make them feel slightly out of place (and remember, the bride is the talk of the day, not the lady with the big, red, sparkling gala dress).
Consider the season you are getting married. If you are getting married in the summer your chances of finding a nice casual summer wedding dress is probably best in the early spring when the new summer season's clothing is in stores. You may find things all year around though so keep your eyes peeled for that special dress as soon as you've decided what kind of dress it is that you want. Check out your favorite designers and stores and you can also search online to get an idea of what stores are offering.