I use a whole lot of different material to make reeds for my tubes.
I've had great success by cutting up surgical gloves like the doctor uses and i've also ordered latex from various call maker supply places like Brookside Game Calls, but i've had the best success by buying latex at my local pharmacy/medical supply store.
The brand name is "Thera Band" and it's used to help you build rubber exercise bands at various strengths for rehab/stretching purposes. It comes with 3 different thicknesses/colors of latex strips about 5" wide by several feet long. There's a ton of call making material in 1 box. Hope this helps