Have you tried talking to Rob Millenship, contact details on p56 of the latest PFA mag. He is a Pitts expert and does some painting.
Mick Allen is good, but usually booker up 6 months in advance. What about Cliff Lovell at Hampshire light planes, Chilbolton. He might be able to do something, but is not a paintshop, more into aircraft restoration. Many of the larger paintshops dont usually deal with fabric aircraft so might be worth seeing what their experience is. Doesnt anyone do painting at whatever airfield you are based at?
Good Luck!