How can you get OSX if your hard drive crashed and you don't have the original DVD anymore?
In the past AppleCare has been able to send replacement system discs.  I would assume the same holds true today but my only doubt stems from the age of the Mac, since it may be unsupported by AppleCare.  It's definitely worth a call though...


Call 1-800-APLCARE (Or find your Apple support number HERE if you're outside of the US/Canada)
Give them your serial number (found on your Mac, underneath your battery)
Explain that you have misplaced the gray system discs that came with your Mac and that you need replacement discs in order to reinstall your operating system.


Hopefully they are still able to provide replacements!