Any musical keyboard with a MIDI OUT socket will do (i.e. practically all of them) but ideally you need an 88-key one. The reason for this is that the bottom two keys are used to change sounds and the top two to change songs. These settings can be changed but if you’re using a shorter keyboard you may run out of notes. You’re welcome to use your own or we can supply either a normal synth keyboard or a master/controller keyboard.
This is a Studiologic Numa Nano - a controller or master keyboard. It doesn't have any sounds or speakers so is cheaper than a fully-fledged synth. However I didn't like the action on it so sent it back.
A VAX 77 from Infinite Response - the world's only folding full-sized controller keyboard.