The flagpole on Speedy is made from a solid fibreglass rod, about 6mm diameter, so kite supplies would do for that.
I'd always been in two minds about flags. Generally speaking, recumbents have much more of an 'ooh' factor than upright bikes, so people tend to 'see' them more anyway - i.e. they actually look, and they take some notice.
However, for riding in the morning commute, or for those days I finish at a time that takes me home in heavy traffic, I've found a certain added 'bigness' generated by a streamer. Not a
flag mind. The original yellow flag had no such effect, but the 1m red/orange/yellow streamer I made up seems to grab people's attention.
Of course, for high days and events, Speedy is mostly wearing a proper Skull'n'crossbones.