höllenteufel,
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.
As far as disadvantages of a pressurized radiator compared to a non-pressurized radiator:
A marginal water pump could leak a bit more. I use the leakless pumps and have not had any issues.
Good Day!