You may be able to find some older Roland, Yamaha, Korg and believe it or not Casio Keyboard Controllers. The new Roland looks pretty nice and has the added benefit of a synth engine built in. Yamaha had two version one had mini keys the other full sized.
I’ve never liked the term ‘Keytar” not sure when and where it came from, In the past it was always strap on controller ( not much better I guess)., or keyboard remote. To me “Keytar” make it sound too much like trying to be a guitar. Nothing wrong with guitars of course. Those that like the term fine, I’ll still talk to you .
Even if you were to get the controller you are looking for and you didn’t care about the sounds built in other than the bass, it will still control stuff like software through midi or your other synths. Like you could get the Ax Synth and the Fantom XR and there is definately killer bass in there controllable via MIDI.