Buying an Older Copy of Corel Software?

I have an old version of Corel Print House (R) 6 and Corel (R) Custom Photo 6. I have been using and created hundreds of files with these programs over the years. After a recent crash of my computer I need to re-install it. I still have got the original CDs, but no serial number! The extension files are CPH which none of the new Corel versions can open! If anybody knows where to buy, I should be very grateful.