Metal Finishes

I am currently using Maxwell and Rhino/Flamingo as my renderers. I am not very clear on how to get precise custom finishes such as Satin Nickel, Rubbed Oil Bronze, ect. It is pretty important that I have these very close for the projects that I am working on and was wondering if there was any advice on where I can get some textures or where I could find the mathematics for custom metals. More specific, are there particular rendering packages (3ds, Vray, ect.) that are better for metals than others?