Who manufactures sheet with one side impregnated with fiberglass cloth?

I apprenticed as a plastic welder part-time. We used white, polypropolyene or polyethylene (?) sheets of various thicknesses to fabricate horizontal and verticle tanks. The sheets were impregnated with fiberglass fabric on one side which was configured to the external side of the tanks. Then additional fiberglass was added to further reinforce and strengthen the tanks. Who manufacturers this product and what are their contact details (postal address, e-mail, phone number)? Thank you for your assistance.

Plastic sheet like corrugated cardboard

I am looking for a source of plastic sheeting which I have only seen an example of once.

It's a lamintate 7.5mm thick constructed like corrugated cardboard. The sample I have is dark grey in colour.