I have seen them on eBay
. Try there, they have a lot of saddle bags. If you haven't done eBay much, sometimes it is tricky to find what you are looking for. You can type in saddle bags in the search but that will bring up all kinds of bags like motorcycle bags or hand bags. You can narrow your search by going to the categories first and then search once you are in the right category.
Horse stuff is under Sporting goods, then Outdoor sports, and then you will find Equestrian, then type in "saddle bags". Then you just get horse related saddle bags.