I assume you have checked if your controller supports one and what inputs it needs, then I'll tell you I have no idea but I'm interested to know what you find.
It's not really a voltage thing (36V, 24V etc), unless you want a combined battery meter. It's more a "hall sensor" or "potentiometer" question.
You can make a reasonable guess at this by looking at the label on your controller. Hopefully it should tell you the throttle voltage input: 1V-4ishV would indicate a hall sensor. Anything else probably indicates a "potentiometer". You can put a "pot" on a hall sensor controller with some extra resistors, but you may have issues the other way around (might work though).
Although not in the UK, ?ecrazyman@gmail.com? (on ebay) from HK sells them and has a good reputation. There's also a Lincolnshire based seller on ebay that sells them. Baring that you have Alien Ocean (seller of the Alien kits) who might sell you one.