You should be able to call AppleCare (1-800-APL-CARE in the USA) and buy replacement install discs for your computer, unless it's really really old. You will need to tell them which Mac model you have.
Or if you're near an Apple Store, call them. They might be able to do something for you.
Master password and administrator password are different. A master password exists if you use FileVault, and in the event you forget the administrator password for an account secured by FileVault. Administrator passwords are used for installing software and updates and other tasks that affect system files. So, is it your master password you've forgotten or your admin password?