There are dealers associated with the Western art trade -- easel art derived from illustrating Westerns -- that buy and sell both posters and the original paintings that were then printed on book covers. If I were this person, I would buy from the regular magazine suppliers (Barnes and Noble or Hastings, for instance) copies of magazines about Western art and then follow out the Gallery ads and particularly the auction ads to make contacts with dealers. Most of the dealers I knew are dead now or I'd give you names. Helen Card was noted for this category, so googling her name might work.
Also, www.askart.com is always a valuable place to begin. It's a compendium of American artists, each one surveyed in terms of works, galleries, and so on. Or you could ask the website directly. If you put in the names of the artists -- though they may not have signed the art on the covers -- you'll probably turn up a lot of leads. Put in John Clymer, Howard Terpning, and other people like that. There are also lists of artists in various categories, though I don't remember seeing any category labeled "cover art." Western writers have opinions on this subject: Richard Wheeler, for instance, froths on the subject of the inevitable gunfighter next to a busty gal with a torn blouse -- none of it remotely related to the story inside.
It'd be fun to pursue this at the very least!