http://www.evike.com/ Has a lot of different brands. Another site to compare models and prices to the other suggestions.
Its been awhile since I did any airsoft stuff though.
There are some great name brands but they can be expensive, there are a few good brands that are just a little more money that the cheaper stuff. (cheaper metal construction, not real cheap plastic stuff)
There are a few Chinese knock off brands that are pretty much copies of the high end stuff. Usually made with a bit cheaper materials and lower QA.
You want metal construction if you plan on rough handling, though some high end plastic guns may be better than a cheap metal gun.
Have fun.