I was going to say something crass like 'after apache have stolen all their horses' but I do share your pain.
Whatever period or scale this always seems to be an annoying problem for wargamers. Dragoons or Light horse only depicted in a single 'mode'. I agree with Treebeard, your best bet is probably a sympathetic paint job of infantry wearing similar uniform. Thankfully 20mm is a forgiving scale in that respect.
I have found contacting the manufacturer directly can be helpful in this kind of situation. They are usually quite sympathetic and appreciate the effort you've made to contact them.