Some designers offer a free stitch guide with each painted canvas. I will list all the designers whose canvases come with a guide below as I discover them.
Barbara Bergsten's canvases all come with stitch guides. You get them from her blog and then pick up the canvas at your local shop. Here's the link. Note that this is a great place to get individual stitch diagrams, not just a guide for one of Barbara's canvases.
Princess and Me specializes in ornament canvases, mostly humorous Christmas motifs, and all their canvases come with stitch guides.