How tall are you looking for and how big of a flag. With that basic info, a mechanical engineer could calculate the loading for standard steel pipe (2 or 3 ") or galvanized pipe. A cap of some sort and a pulley system and you've got a flag pole. I think you can get the forces generated by a flag at various windspeeds off the web for the calculations. Just a thought.

TxEngr (I'm a sparky, not mechanical)