How to make a ~90 mm diameter 50 cm forearm roller as cheaply as possible?
Since I live in a region where millimeter measurements aren't common (yes, that region) the first thing I did was convert to inches to get an estimate of size so that I could look for something. I noticed 90mm is very very close to (approximately 1mm larger than) 3.5 inches.
3.5 inches happens to be the outside diameter of pipes (plastic/pvc and steel) that are nominally 3 inches (inside is approximately 3 inches).
Since you're looking on the Internet for pipes, you may be able to buy US stock (either from a US outlet, or from a manufacturer that makes goods overseas for sale in the US). I don't know that 88.9mm would be a common size outside the US market though, so if you're not able to purchase these online you'd have to find something completely different.