fork oil definitely deteriorates over time...and I don't believe you can purchase a new, complete fork assembly (meaning assembled, ready to install). You can certainly buy all the parts but it doesn't appear that you can purchase the assembly.
A typical fork rebuild consists of replacing the oil and dust seals and fluid, while inspecting for deterioration of any other parts. Of course, like most things, I've never done it myself...but I have watched it done on youtube!