There arte no legal ways to download Mac OSX, unless you have a very new machine that loads fromthe network (wuch as some macbook airs).
You can call apple and beg; you cna buy a used copy or you cna buy a new copy.
Sorry, but that's how it is.
What happened to your copy of OSX that's on your mac? Still running i hope?
I would make a complete, external boot-able clone.
Don;t knwo how? Buy a good book on mac - i suggest 'Mac OSX the missing manual" by Mr. Pogue.