I don't quite understand why you need it. Is the mic stand 3/8"?
Is the shockmount 3/8"? If so, you'll need a 3/8" to 5/8" adapter, which is a whole different animal.
In either case, you might be able to get one in the plumbing department of a large hardware store, such as Home Depot. They have a lot of different adapters for copper tubing and pipes.