Euro Car Parts are excellent, I'm down there so often they put me on a trade account, they sell *everything* for the K11, everything. Give those guys a go if you got a local one, they are very lenient with prices if you wiggle a bit. Besides that there's others, though I don't know any as widely around as ECP.
The quality of the parts is up to or exceeding OEM standards, often what you buy is of great quality, I've bought a clutch and brake pads etc just general service items and everything went on as it should, no need to play with things, it was clearly made to a standard.