You'll have to import one from HK or buy second hand as there are no KWA retailers in the UK at the moment. Anything any shops have is just old, unsold stock. You'll be very lucky to come across one if you do.
Mags are also hard to get hold of for the same reason. I'd either get the plastic TM version, or make do with a metal version that won't work when it's cold.
KJW and WE are both full metal and quite cheap.