I think any MIDI controller that can send CC. There are several decent ones out there. I presently use a SoftStep II and a G Labs GSC-4. I think you have to figure out how much you are willing to pay and what footprint the MIDI control board takes up on the floor. Are you going to be controlling other MIDI capable pedals as well? Some users report being happy with their Behringer FCB 1010. Here’s forum and other links to this topic:
BB specific external midi controller? and
Please Help! MIDI Setup with Boomerang III and MIDI Controller and
https://reverb.com/marketplace?query=MIDI%20Foot%20Controller&page=1
I think that it’s also highly dependent on what you might already own. For example, some users use OnSong or SetListMaker from their iPad. Keep in mind that you have the option of using a software looper so that you’re not tied exclusively to a a h/w solution. If you’re patient, you can wait to see how the BeatBudddy user community reacts to the MIDI Maestro when it’s released 