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.
Old World Designs (California)
Needlenook of La Jolla (California) - They might only do guides for canvases purchased from them, however.
Quail Run (Arizona)