Tutorials | Buchla Easel V - Episode 1: Overview 

I see your question wasn't answered but almost certainly he was using a midi controller. I've seen it work perfectly with various Arturia and Korg controllers. I haven't tried but I think some of the switches can be remotely controlled with a midi-mapped knob or fader and the switches that only have two states, could probably be mapped to a button. I would try to find something with knobs, faders, and buttons just for compactness. Faders are always nice since it's easier to handle a few at a time but a lot of knobs are not things you would tend to actively adjust ongoing in performance, the easel is so smart the way it's designed to be played. Also, there are 29 total knobs + faders and what looks to be 9 or10 switches that are more than 2 states possible, plus 6 more that I think only have 2 states (so potentially mappable to a button). If you find something that works well I'd love to hear about it.