These keycaps are such niche items that you won't generally find them just sitting around in some webstore's inventory. You typically have to get in on a group buy or find them for sale, used, by someone who doesn't want theirs anymore.
One problem with the secondary market is that many keysets are priced as collector's items. That will double, triple, or more the price you pay. And anything that is priced reasonably by a fellow member of the community will have many interested buyers; getting your PM to the seller ahead of anyone else can be challenging.