You have Dymag who will do a sterling job, not sure where they are based but it is in the south, or you could ask Colin at Oxford Industrial Finishers who will also do a very good job (01865 882883).
I had a 180 on my mag 6 inch when i got the bike and it was awful...the narrow tyre is stretched across the rim width giving a flat profile that , even though the tyre was new, it behaved like a really badly squared off tyre...snaked all over white lines/overbanding and was a sod to turn.
190 and it was transformed...the proper tyre profile.
The ideal fitment would be a 190/55, but there aren't many options in that size apart from supercorsas or the new dunlop 209 gps for example.
Got a set of mags for my 996 with a 6 inch rear, was told 190 section was the right size. Sent them away to Dymag for a referb too £220 but includes a check over for saftey and peice of mind address and details here