How do you make Ketchup Doritos?
I don't have a recipe, but if you had say a dehydrator, ketchup, and whatsit--dextrose?--that helps dry things resist clumping, you could make your own. I believe Ferran Adria carries a powder product, on his website albertyferranadria.com.

Add the powder to your own tortilla chips that you've just fried, and should work. 

Alternatively, multiplex movie theatres often have several different shakers of flavourings--cheese, sour cream/onion, that sort of thing. Perhaps Costco would carry something similar.