I'm approaching this one with some caution, because it's made me doubt my own sanity. But honestly, I have never in my life heard of regional locks on software disks. Video DVDs yes, but software? Software is completely region free - there simply is no restriction on installing it on any machine, regardless of source.
As far as CS3 goes, when I bought a copy in Japan, I got it direct from Adobe Japan. They had to import it from the U.S. themselves to provide an English language version. Personally I would say buy it from a reputable store, or a company with a very high rating on eBay. Read the customer feedback and be absolutely sure you're getting what you're paying for. eBay is an absolute minefield when it comes to software sales.