If you are looking to buy one, the Racing Beat Type II is a similar design, and it would be a lot easier to find. The RB ones are still available new and they pop up in the classifieds every now and then. The HKS are out of production and I don't recall ever seeing one for sale. http://www.racingbeat.com/Miata-1990-2005/Body-Aero-Components/57104.html
Yahoo Japan is the best bet for these sort of things.
It's best to go through a broker to avoid the risk of customs/postage etc, that and I'm not sure people outside of Japan are permitted to bid on yahoojp.
Import monster are a common choice.
I often go through jdmparts.rupewrecht.com The owner is a great guy, and always fair.