In the US this is available from the LHS, and is usually 2 oz/sq yd woven fiberglass cloth. He's right, 9mm is way too heavy. 3mm would be plenty for a .25LA, and even 1.5mm is not too light if you used 3mm frames. What are the dimensions of the boat? Airboats with that power level are usually in the 600-700 mm range.