Have you searched MatWeb? This is all I found under "fiberglass polyester resin". Not sure if this is exacty what your looking for but it may be a good starting point.
Check with the supplier. Then validate your model with that as the starting point. From my experience vendor data has been over 50% off in the past.
Remember your part is probably not isotropic. It is probably transversely isotropic because few fibers will lay perpendicular to the mold.
I hope this helps.