amc and atc hire ridgerunners, have website. amc also hires for many backcountry positions in the white mountains. the SCA is a huge organization that distributes young people to non profit, national park, stuff like that, all over, all over. only thing about the sca is that they only pay a stipend, just enough for you to choose one: food or beer. but the sca is a great, great first step.
otherwise i am not aware of any central web site for guide positions, through i would love to know of one. all the guided groups i have met have been very small time and local, and so i imagine finding a position with one would require some phone book type research. but one will lead to another, you know if they are not hiring they will know of 5 other places that might be. and so on.
whatever you do, dont expect a 401K!
ridgerunner milo