If you're in the states check out Lowes, Home Depot or any building store. If you know anyone in the construction business maybe ask them if they have any extra or where they get theirs from (maybe can get a deal)?
in the staging industry, we use schedule 40 steel pipe. not really sure where you can buy it tho. i'd check out some commercial construction companies or something. not really sure if home depot, etc would have stuff like that.