You may have to do what others have done: Buy some lightweight aircraft covering and paint it yourself. Sig sells Coverall and has the appropriate colored dope You may want to check with Aircraft Spruce and Specialty: Pilot Supplies and Aircraft Parts from Aircraft Spruce
Hoping someone with experience in working with these fabrics will chime in here.
Loosing the supply of Solartex is a PITA so until or unless someone else decides to take up the cause. Modelers are going to have to switch thinking and use conventional covering which is more work but what can we do?