What did you end up using?
Honestly, I didn't end up using anything. I stopped playing the game. Software Options
That being said, if you just want to play with a controller and don't care about multiplayer, you can use any of the plenty of joystick mappers out there, which is what I did before I quit.
There are also some apps out there to convert DirectInput to virtual XInput if you're set on getting multiplayer working, but I don't have any specific suggestions since I don't run Windows. Hardware Options
If you're up for buying a new controller that's good all-around, the Logitech F310 is a popular choice with a hardware switch on the back to toggle between DirectInput and XInput modes. There's also the F710 (at about twice the price) if you prefer wireless, though I've never liked having to replace the batteries.
It can be a little hard to get your hands on an original XBox 360 wired controller that's still in good condition these days. And they're not quite as portable, but they're certainly comfortable to use. Most of the knockoffs should work just as well for your purposes, though. Rant
I don't appreciate vendor/tech lock-in, especially when libraries like SDL make it a cakewalk for the developer to support alternatives.
Nor do I appreciate the notion of "controller support" requiring unofficial, unsupplied third-party drivers, as was the case for MacOS.
They should have supported both XInput and DirectInput, used a cross-platform controller library, or at least listed on the game's page that there's no official controller support for anything but XInput -- which they still haven't done a year and a half later. Edit: Moved the rant to the bottom to bump potential solutions higher in the post.