I just had a quick check on Amazon, wow! I have the original series (minus vol.7) from the 70's, it quickly sold out, and was reprinted in single volumes in the 90s (1994 I think) and again, quickly sold out. There are some single volumes for sale on Amazon ranging from about $30US up to $94US each!
Search by title or author, Andrew Mollo has written some great books on the SS and W-SS, some are available quite cheap, his excellent Pictorial History of the SS is in there for a good price. Or try ebay on world-wide option search, not just ebay.au and see what happens, or keep it on permanent search. But definately try and find the single volume, waaaaay cheaper!