Weirdly, I thought there would be more mounted figures that are separate but looking around I see your dilemma.
One imperfect solution, if you don't mind some conversion work, is finding models where the legs are spread apart ( like this one) and tweaking or bending them a bit to make them look like they are straddling their mount. The example I linked is a Reaper "Bones" figure because being made of PVC, it would be easy to clip off bits that would keep him from being seated, and you could probably soak in hot water for a bit to make it easy to bend the legs a bit if needed. Metal figures you can gently bend with needle nose pliers (be careful not to scratch the metal).
You could also buy halflings, clip off the legs, and then glue the torsos on the seats, then gluing and repositioning the legs to make them look mounted, and finalizing the look with some sculpting putty. Don't know if you want to put that much work into it though.