Off the top of my head I’d say: ARMA: Association for Renaissance Martial Arts is your best bet. These guys are based around reconstructing European martial combat styles. They mostly do swords, but you never know. I’ll let Starke come take a crack at this one once he’s done working on the dryer.
There’s nothing I can say about fighting with a mace on horseback. In general, neither of us know a lot about mounted combat. On foot, using a mace isn’t that different from a hammer or a maul.
I suspect, effective mace fighting builds off the same mechanics as axe fighting; build as much momentum as possible, with the least movement necessary. But, remember that conditional, neither of us are trained in mace fighting, so I’m guessing here.
I wouldn’t normally recommend a video game, for obvious reasons, but, the Mount and Blade series has pretty robust mounted combat mechanics, so it might be worth a look, so long as you remember to take what you find there with a few bags of salt, and do some research to back up what you find.