Those ones, which you can get pretty much anywhere. No particular brand as far as I know. Normal piezos work too, but sound different.
RedSpring Ambient Spring Synth
Blindsmyth
#22
I'm also feeding (self built) piezos wich are basically the same as your bought once into my axoloti(s).
It sounds ok but you loose a lot of low end and get a rather harsh sound due to the impendance mis match. When you run them through the right preamp you can a much more even sound. But maybe tha harsh squeaky sound works rather good on your instrument 