I don't know if you have checked at the below three sites, but I gave the direct links for you to check. Let us know if you want suggestions for more places.
This is the link to our western ebooks (mobi / prc for the Kindle)
https://www.mobileread.com/forums/ebooks.php?s=&sort=ebook&order=asc&page=1<r=&f=1 28&genreid=western
This is the link for westerns on the feedbooks site.
http://www.feedbooks.com/type/Western/books/top
This is the link to the westerns on many books
http://manybooks.net/_scripts/categories.php?code=WES&s=1