You can get a good deal on a steel pipe online for example metals4u offers cheap but good quality steel pipes. You can also find them at many scrap yards for an amost non-existent price.
I found that the Pipe Friendly Tape Measure by tekforce is far more accurate and cuts the hassle.
China is a large country to export steel pipe, if you want to buy steel pipe, you can import from China.
Australia makes steel pipe and tubes
Brazil exports steel pipe and tubes
Brazil makes steel pipe and tubes
California makes steel pipe and tubes
China exports steel pipe and tubes
China makes steel pipe and tubes
Illinois makes steel pipe and tubes
Japan makes steel pipe and tubes
Mexico exports steel pipe and tubes
MS pipe refers to mild steel pipe and ERW pipe is electric resistance welding pipe. The difference is ERW is made of very fine hard steel, while MS (black pipe) is made of malleable steel.