Phred,
Go to Brasswork's web site:
https://www.thebrassworks.net/
Look under the heading of Parts and Pieces; you will find overflow tanks. I use the 15" stainless steel in my cars. The 13" would be large enough if you have a space problem.
At least one size of these same overflow tanks are available from Snyder's.
I mounted one on the firewall the other I mounted on the right side of the radiator.
Good Day!