If you want some lightweight fiberglas you can find it at any lumber yard.
They call it Drywall Joint Tape. It comes in rolls, typically two inches
wide
by whatever length they happen to sell


Getting kind of high tech here aren't we?G

What's wrong with good old fashion paper for the gussets? Was good enough on
my old Aeronca. 65 years young and the gussets are still doing their job.
Lots of suitable paper out there to replace the "fish paper" Aeronca used.
Pink roofing paper - a roll of it should make a few hundred planes worth of
gussets. Or you can use the left over to make model rocket engine cases if you
wish. Nice big ones too, not those wimpy things Estes sells. Water color
paper comes in weights that make good gusset material. Even heavy Kraft paper
works. Grocerys all come in plastic now so you may have to buy it too. And if
you must spend the money for the same stuff that came in the certified Aeronca,
take a look at some of the gasket paper sold in about every auto store.....