I would not depend on the forearm trick, most experts agree it will not give proper measurements.
A shotgun coach might be able to help, but they usually charge for stockfittings. Gunsmiths may or may not know how to measure for proper fit.
Some places use a try gun, which has a special stock that can be adjusted for LOP, Cast and several other measurements. Fittings usually go for $150 up and do not include alterations.