Do you mean 20mm diameter? ID or OD? I can't remember the actual diameter of 22mm plumbing pipe, and as I'm currently sat in bed in a rented cottage 200 miles from home, I don't have any references with me! I'm only likely t have smaller stuff for piping, or larger stuff for boiler barrels in stock, and possibly a bit of secondhand 22mm, but can't be sure from this distance.
Model engineering suppliers such as Blackgates engineering do sell in short lengths, but there is precious little obvious logic in the range of sizes available.