I know they're in England, but you could try contacting Ravenair at Liverpool Airport. They have a large fleet of PA38's (13 I think) and they do their own maintenance on them so they might be able to help you find the part you need.
All contact details on their website:
Ravenair - Flight Training, Air Charter, Aerial Survey, Aircraft Engineering, Aircraft Handling, Aircraft Hangarage, Aircraft Management, North West England, Liverpool Airport, City Airport Manchester (Barton Aerodrome)