10.4Leopard Operating System
Can I buy 10.4 Leopard form Apple?
iMac, OS X 10.11
Question marked as
No, they stopped selling Leopard DVDs a long time ago*. You can ask them but unless they find one under the stairs somewhere they won't have one.
If you get one on the open market make sure they are not grey colored discs that came with a computer. They only work with the model with which they were supplied. Get the black ones.
*10 years ago they told me I bought the last Snow Leopard server DVD they had for sale.
If something newer will work, Apple has some versions available for download here How to download and install macOS - Apple Support