I know problem solvers does some but they're pricy. The ones I got I bought from a Danish site so not much use to you, but there's plenty of ones available in the bmx circles and I know sjs used to stock some. Just look up 1 1/8 to 1 inch headset reducer rings/shims and it'll probably find some UK results but I'm only getting eu and DK
About £25 for Wheels or Problem Solvers.
Look a bit horrible though - can't you sell the fork and get one the right steerer diameter?
Or the frame for that matter