Here are a few popular online catalogs of harp music books for all skill levels. Method books, solos, duets and other arrangements for lessons are available from these sites if not from local music stores.
Harp.com - in association with Lyon & Healy, one of the largest harp catalogs.
Vanderbilt Music Co. - a top source for all harpists.
Melody's Traditional Music & Harp Shoppe - a well-rounded collection of music for all harps.