Sarah McLachlan is an artist included in the Ectophile's Guide to Good Music The Ectophiles' Guide to Good Music which I came across a couple of years ago. They talk about mostly female adult alternative artists, and as a fan of Sarah McLachlan, I liked a lot of the ones mentioned.

Another site you might like is Music Roamer. Music Roamer - Discover New Music You type in the name of an artist, and it gives you a cluster of similar artists.