Where can I purchase a 6 inch round diameter metal plate or disk for a experimental shaft running at low rpm

Would an aluminium pie plate work for you?

Home Depot

If you can't find anything locally, McMaster-Carr (http://www.mcmaster.com/) is a good source. Go to Raw Materials \ Metals (http://www.mcmaster.com/#metals/=b9flcl).


Here are two options, for example:

http://www.mcmaster.com/#steel/=b9fje7

or

http://www.mcmaster.com/#aluminum/=b9fkk8