Needlepoint.com

One online shop lists the canvases that come with their own stitch guides that they have for sale.  These are from varied designers.  I recognize Kelly Clark, Princess and Me, A Collection of Designs, and Shelly Tribbey.  Prices vary from $5-15 extra for the guide.  Note that all Princess and Me canvases come with a stitch guide included at no extra charge. I don't know if the stitch guide alone is available for the other designers' canvases but one A Collection of Designs canvas comes with a guide at no charge.
http://www.needlepoint.com/index.php/all-canvases/canvases-with-stitch-guides.html?p=2

I wish more online stores were organized this way.

UPDATE: Needle Works in Texas also has a category for canvases with stitch guides. It looks like these are all canvases that have the stitch guides included in the price but I can't be 100% sure.  Here's the link-
http://theneedleworks.com/catalog/index.php?cPath=27