I am a boilermaker by trade and have worked in a lot of w/shops here and in NZ , most large w/shops have a lot of scrap pipe an piles of old flanges , of many different sizes just lying around .
Manav's list is a good one his tube is 200mm diameter schedual 80 carbon steel pipe , 150mm sch pipe is 168mm o/d and 100mm sch pipe is 114mm o/d , there are hundreds of off cuts out there if you ask , don't buy new .
A large 12 inch , 150 lb blind flange would make a perfect base about 450 in dia. and its already drilled so that saves labour/costs .
Shop around you might be surprised as over the years I have seen lots of us astronomers given old steel like this for free when the owner/boss discovers its for a telescope .
Brian.