Q: I bought a used iMac G5 A1145 and it didn't come with installation disc.  I don't know the admin password so I am unable to do change, delete or add new accounts.  All the previous users information is stored on here.

If in the US, you can call 1-800-MY-APPLE, ask for a operating system specialist, then tell them you have lost your system install/restore disks. You must have the serial number at hand for them to see if they have the right disk set. There will be a nominal charge it they can supply disks, but it's far cheaper than paying the inflated prices for Tiger or Leopard on the used market.

First you should ask the orig owner for the disks and the password.

This post was down a few from yours.

https://discussions.apple.com/thread/4842660?tstart=0

Restore Tiger 10.4 & Leopard 10.5  DVDs are available from Apple by calling 800-767-2775 as of January 20, 2013.

https://discussions.apple.com/thread/4720126?tstart=0

Robert