There is a steel supplier of Barry st Bayswater, but any steel supplier has this, some are reluctant to cut down stock, but from memory this bloke had some surplus.
Two ways you can do this, steel tube is 48.3mm OD, steel rod (solid) is 50mm perfect for Olympic plates but quite heavy, I have both.
I bought threaded rod from bunnings in the plumbing section, the nuts they have, but it is wise to go to bayswater bolts and also buy four nylock bolts...two for the top of the handle and two for the bottom.
I took my handle to an engineering company to mark the knurling on the handle, most lathes have a knurling wheel. Tell them to make the knurling deep.