Try giving Apple a ring to see if you can purchase a replacement set of discs. Have the machine specs including the serial number ready when you ring.
There are 2 answers to your question, first is as harryb2448 call apple to get the replacement disks, one for o/s the other for applications. These will be dedicated to your computer only. cost = $16 each in US.
the other answer is get snow leopard this will only let you install o/s part, then buy life 11, the latter may not be necessary if you just want to install the o/s.
Better still buy the mac box set may cost you over $100 US, this will have snow leopard, ilife 11 and iwork. Break down s/l is $ 32 or more and Ilife 11 also over $30.
Take your pick. if you just need the restore disks, then it is not flexible as buying the snow leopard, where you can use on future macs.