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

Paid Custom Stitch Guides from Online Shops

Some online shops will design a custom guide for you whether you bought your painted canvas from them or not.  Here's a list, in alphabetical order.  Most of the shops listed don't give much detail so you will have to contact them and talk over what you need and how much it will cost. I will add to this list as I find more shops that offer this service.  Buyer beware!  I don't know how detailed or good any of these guides are.

NeedleHouse in Houston doesn't mention prices or any other details.
http://www.theneedlehouse.com/jm/index.php?option=com_content&view=article&id=89&Itemid=91

Old World Designs (California)
http://www.oldworlddesigns.com/

Needlenook of La Jolla (California) - They might only do guides for canvases purchased from them, however.
http://store.needlenook.com/Custom-Stitch-Guide-Service-p36795.html

Rittenhouse (Pennsylvania)
http://rittenhouseneedlepoint.com/stitchguide

Quail Run (Arizona)
http://quailrunneedlework.com/services/stitch-guide-design