You will find details of WPA members who offer treated wood products and associated treatment services in each of the relevant sections of this site:
Here we list additional WPA members under their relevant 'categories', starting with Manufacturers.
Trade, Technical and Learned Bodies
Personal (Independant Consultants) - ask the WPA for contact details
These independent consultants have experience and competence in advising on wood preservation and may be able to offer expert witness services.