I have a few places I love to go and peruse the art supplied, assemblage materials and collage items. Some of them I like for just one item; others, everything on the site screams "You NEED one of these". Probably my all time favorite is BMUSE, though I do not get the chance to purchase much. Vicki "Mack" is just one of those really nice people I would not have met witjout the net. I love Anima Designs for the objets de' relgion and super kits. The bottle necklace kit is great, and comes in a cool little bag. Gift registry there, too! (hint) For a reasonable prince on clips of all kinds, see Bitsnpieces4U. I do not gravitate towards scrapbooking much, but I do dabble. Every once and awhile I see a paper pattern or paper maker that could be used in artwork. Such is the Wildflower line by Cactus Pink (love the name!)
Even more wonderful are the testural, thick inked sheets hand screened by Hambly Studios. Their transpaercies are yummy. Speaking of transparencies, I am in total love with the Christine Adolph collection from Creative Imaginations.
For ATC products and books, check Poet's Dream.For books you do not see many places (good for ideas and such) look at Create and Treasure. (Though it does have a scrapbook bent, the overlap between altered art is obvious.)
There are a couple of more shops that have everything: Thieving Magpie, Sacred Kitsch, Sweetwater Sisters & Silver Crow Creations. I hope you find something you like!