Paddocs & Craddocs both do them; either original chrome, or as some-one has mentioned 'black teflon' coated versions. They are around £50 a side, and will ship mail order; difference twixt the two is pennies, and I doubt you will find cheaper, and both are usually bearmach or britpart brands, which is likely what you would get from a franchise main stealer for 50% more wanga!
teflon balls seem to be about £10 more expensive; and from reports they are a bit 'lighter' on the steering, and less inclined to pit. But I dont know any-one who has had them on a landy long enough or worked hard enough to give realistic long term comparison; crome ones last fifteen to twenty years or more, so.......... your call.
