I don't want to sound too much like the attorney that I am, but if you purchased your Macintosh new, you purchased not just the hardware, but a *license* to own and use the software on those disks.
If you have lost those disks, you still haven't lost the right to that license. You still have a legal right to access to that software and Apple is legally obligated to make sure that you have that access.
What that means to you is that if you can prove to Apple that you purchased your Mac new (i.e. with a store receipt, or a credit card bill) you have a right to have that software replaced at no additional charge, though they are entitled to charge you a nominal sum to cover the cost of the physical media and shipping.
If you approach Apple and are really polite, and can show unequivocally that you purchased your Mac new, and explain the above, I've known them to just send a user replacement disks.
I can't guaranty that it will work that way for you, but in a perfect world, that's how it is supposed to work.
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Randy B. Singer
Co-author of The Macintosh Bible (4th, 5th, and 6th editions)
Macintosh OS X Routine Maintenance
OS X Maintenance And Troubleshooting
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