Ahoy there Red Maria,
I would think that flags were made out of what ever material was available and could be found on hand for that time period. Like maybe a black wool or linen blanket, stitched with a bit of sail cloth for the design. That would probably be fine for the less desiring pirate, at least until some stronger weight canvas and a good seamstress could be found a shore.
I actually want to make a pirate flag me self, don’t have the design just yet.
I'm egar to see what you do though! If you don't mind sharing, it would be great to see some in-progress pictures of what you end up doing.