I use ambient music a LOT in worship and learning settings, and have acquired a bunch of music over the past few years thanks to the recommendations of a number of friends and my own exploring. Here are some great musicians to start with. The styles vary from no beat to more upbeat, from fluid to more jarring, but all excellent.
Album links are to the Australian iTunes where available (or alternatively Amazon or some other band site). If you're in another country, you should be redirected to the relevant iTunes store for your country.
Quite a few of these came from emusic.com where I have a monthly subscription, so they're relatively inexpensive. Sadly, emusic doesnt currently accept new Australians subs due to local recording company agreements.
John Foxx & Harold Budd - Translucence / Drift Music
Harold Budd - Agua
Eraldo Bernocchi & Harold Budd - Music for "Fragments From The Inside"
Jon Birch and Johnny Baker - generally laid-back beats (go for the instrumental albums, although the others are worth hearing too) - at Proost
Brian McBride - "The Effective Disconnect"
Cluster - "Qua"
Dawn Landes - Dawn's Music
Efterklang - "Tripper"
Goldmund - "Corduroy Road", "The Malady of Elegance", "Two Point Discrimination"
Hauschka - "Room to Expand"
Hauschka and Hildur Guonadottir - "Pan Tone"
Helios - Eingya
Hilmar Om Hilmarrson & Sigur Ros - "Angels of the Universe"
Jonsi and Alex - "Riceboy Sleeps"
Labradford - "fixed:content"
Moby - "Hotel" B-Sides bonus CD (and bits of his other work)
Oneida - "Oneida", "A List of the Burning Mountains" (disturbing soundtrack stuff)
Hope this helps you.