The flags are the standard Annin 3x5 nylon flags that anyone can buy at a department store. They weren't even obtained on contract; some JSC administrative assistant just went down to Montgomery Ward on her lunch break and bought them.

Nylon is a great material for weight. Cotton flags would have weighed too much and been too bulky. But as with most polymers, the intermolecular bonds are affected by ultraviolet light. After 30 years' exposure to ultraviolet, the flag fabric is likely to very weak. Sadly we know that Apollo 11's flag is lying in the dust at Tranquility Base. But those that are still standing might be too flimsy to hold up their own weight, or they might disintegrate the instant a future astronaut disturbs them.