Hi Luke,
I normally get this kind of stuff from RS Components http://www.rswww.com, but you're right it's a bit pricey, although quite how much depends on the port sizes you want. I recently noticed Pneu Store http://www.pneu-store.co.uk, they sell SMC parts. I'm afraid I've not had a chance to check their pricing yet but it may be worth a look. I bought a 3/2 valve to run truck air horns from Duncan Rogers Engineering http://www.duncanrogers.com/ who have a good online shop. That cost about £45 a couple of years ago.
Be interested to hear what you find. Good luck.
Michael