LOL. I am still making them! It's a fundraiser for my son's baseball team. I found a few places that sell rip stop nylon--Harts Fabric has some.
The best source I found no longer is carrying it (on-line) that is.
You might try Hancock Fabrics. They had some on sale but not the colors I needed. It was around $6/yard. It comes in 54, not our usual 32 like cotton, so keep that in mind if you order. I made 16 flags from 2 yards (I doubled the fabric as my background is white and a single layer looked too light.
I bought wooly nylon to sew the actual flag and found 100% matching nylon for my appliques. It took me a heck of a long time to get the tension right to sew the applique. The flag part was a breeze..
Have fun and let me know if you need anymore help.
Julie