Many of Ruth Schmuff's canvases come with a stitch guide included in the price. You can see them all on her Tis The Season website. A canvas that comes with a guide will be marked "complete with stitch guide" in the description you get when clicking on a thumbnail photo that interests you.
UPDATE: Ruth has updated her store website (Tis the Season is her painted canvas distribution company) with a list of available guides. Some are for her canvases and some are from other stitch guide writers.
Several folks also write stitch guides for Ruth. You will really enjoy viewing Sheena's work on the Computer Pixie.
Ruth also writes stitch guides for classes she teaches at her shop. These guides are for other designers' work.
Here's the latest guide from Ruth's shop, from Brenda Hart. The canvas is from Eleanor Grosch, one of the graphic designers whose work Ruth distributes on needlepoint canvas.
Ruth occasionally mentions stitch guides for her designs on her blog. Here's an example.
Melissa Shirley has just added a profile of Ruth to her MSD website since Ruth not only writes guides for Melissa's canvases, Ruth also occasionally uses a MSD canvas for a class taught at Ruth's shop.