Have Power Mac G4 with no disc, what OS do i need to buy, and where?

All Power Macs can run Tiger OS 10.4.x & some can run Leopard OS 10.5.x.

Tiger install DVDs are in short supply. Tiger is no longer available at the Apple Store but may be available for $129 by calling Apple Phone Sales @ 1-800-MY-APPLE (1-800-692-7753). For other sources, do a Google search for MA453Z/A (10.4.6), MA190Z/A (10.4.3) and M9639Z/A (10.4). Also look on eBay. Be sure & buy a retail version () and not a model specific version (gray) (See http://discussions.apple.com/thread.jspa?threadID=1976068 Can I use another Mac's system restore disc on my Mac?.

Also check these web stores: http://www.lacomputercompany.com/cgi-bin/rpcart/index.cgi?command=dispitem&type= sku&sku=19521
http://www.allmac.com/shop/index.php?cPath=5_10003

Possible Alternative if you are currently using Panther 10.3 – Do a Google search for Tiger 10.4 Upgrade DVD . It will upgrade your 10.3 to 10.4, but it doesn’t contain the full retail Tiger installer. Should be cheaper than the retail Tiger install DVD.

After you install the base 10.4.x, update to the final Tiger version 10.4.11. Here's the link for the 10.4.11 combo update http://support.apple.com/kb/TA24901?viewlocale=en_US

Look at this link Mac OS X v10.4 Tiger Installing Software Support Page

http://www.apple.com/support/tiger/install/

You can also download the Tiger Installation and Setup Guide

http://manuals.info.apple.com/en/Tiger_Install_Setup_Guide.pdf

When you upgrade the OS, do it via Archive & Install. See About the Archive and Install feature http://support.apple.com/kb/HT1710

Be sure to select Preserve Users & Settings. Also, repair permissions before and after the install.

Mac OS X 10.5 Leopard installation system requirements

http://support.apple.com/kb/TA24950

Leopard is no longer available at the Apple Store but may be available by calling Apple Phone Sales @ 1-800-MY-APPLE (1-800-692-7753).

If you can't obtain a retail install DVD from Apple, look on eBay or Google the installer part numbers to possibly find at an on-line store. Here's what to look for:

MB427Z/A Leopard 10.5.1 install DVD

MB576Z/A Leopard 10.5.4 install DVD

MB021Z/A Leopard 10.5.6 install DVD (single user)

MB022Z/A Leopard 10.5.6 install DVD (5-user family pack)

Installing Mac OS X 10.5 Leopard

http://support.apple.com/kb/HT1544

Mac OS X 10.5 Leopard Installation and Setup Guide

http://manuals.info.apple.com/en/leopard_install-setup.pdf

After you install the base 10.5, download & install the 10.5.8 combo update at http://support.apple.com/downloads/Mac_OS_X_10_5_8_Combo_Update

The DVD should look like this

Caution - Leopard does not support classic mode. So, if you currently open OS 9 apps in classic mode, you won't be able to do this if you upgrade to Leopard.

 Cheers, Tom 😉

****... that was the best reply I've ever read. Apple should reimburse you for your time.