There are several choices of fabrics for wallhangings, banners, and flags. A great source is rockywoods.com. I have no affiliation to them.
You can use outdoor canvas that is typically used for replacing fabrics on patio table umbrellas, cushions, canvas swings, etc. There are tutorials online.
You can use 30D or higher ripstop nylon. This is very similar to the lightweight fabric that 'store' garden flags, etc are made out of. These fabrics do come in a variety of UV coated colors.
You can use 400 Denier High Density Coated Nylon Fabric. This fabric is a heavier weighted fabric than the ripstop, but is perfect for making outdoor flags, banners, and wallhangings.
You can use 160D Cordura Nylon fabric. A lightweight nylon fabric.
Rockywoods offers samples. Click on the link at the bottom of the page.