How specific is a "specific" size? Wealden Tools do rebate bits that can take a variety of size bearings to make whatever size rebate you want in 1mm increments. I think NLS still do custom made bits but they weren't cheap and they're probably still aren't cheap.
I personally wouldn't want to spend £100+ on a custom made router rebate bit and would rather use something more standard and factor that into whatever I'm designing.
The sharpening places here, the ones that can do metal tooling in addition to woodworking, can modify an existing bit to reduce the diameter or width for less than a scratch made tool. It is still better, as the boys have pointed out, to find a bearing/bit combination from off the shelf.
Yes, change the bearing.
But if that really doesn't work, I remember Cromwell grinding a straight cutter to a taper, back in the 90s. It was not cheap, but I wasn't paying the bill and it was what the customer wanted. I think that Cromwell are all over the country, are they not?