Marko,
check some of your local machine and fabrication shops, and ask them for drops around the size you are looking for, drops are smaller pieces leftover from other jobs. You might not get exactly what you are looking for, but sometimes you get lucky. Also ask for schedule 40(1/4" i think) well casing. Its basically 1/4" wall steel pipe and works perfect. Don't be too choosy on your bottom plate either, other than bigger is better and more stable. Check out drops for the base plate too. Tell the guys what you are doing and they should be happy to help. I have a couple of shops that I get stuff from now and then, and sometimes they toss some of the smaller pieces in for free. Remember they are leftover pieces and they are usually willing to let them go for cheaper, and if you get drops, just take them as is. It will cost you more if you ask for cuts. You may have to hit two or three of them to get what you want. I think you can get the tube and base plate for around $100 and maybe a lot less. My best haul was a 4 foot section of I-beam for $75 that weighed 300 lbs.