Hi Boy Nigel,

The Boss FS-5U momentary switch will work as a single AUX switch for the H9. Make sure to get the momentary switch, not latching, and set it to "normally open" with it's polarity switch:

http://www.bossus.com/gear/productdetails.php?ProductId=57

For a smaller form factor, T1M makes a single momentary switch:

http://this1smyne.com/shop/mt-mini-tap-tempo-switch/

This one from T1M hasn't been tested by Eventide, but in theory, it should work. Make sure to order the "normally open" version if going this route.

I hope this helps!