I have had bags made to order by Streifeneder in Germany. Fifteen gallon bags for $350'ish. He needs length, width at root and tip + fill tube diameter.
BTW, he owns the rights to all Glassflugel ships and has parts, like landing gears, etc. He has also sent me 92110 fiberglass cloth. Good guy to knowinfo(at)streifly.de
JJ