Can I buy a copy of Leopard 10.5 on DVD from Apple

Why? (both are "ancient" in computer terms)

You will not be able to get "new" apps for Leopard either

I have a Powerbook G3/500 running OSX 10.4.11 Tiger that works the same as it ever did with all the apps I used back then with the exception of web browsing -

I have mitigated that problem to a large degree by using

TenFourFox: A fork of Mozilla Firefox for the Power Macintosh and Mac OS X Tiger PowerPC

http://www.floodgap.com/software/tenfourfox/

Have fun.

It's sorta like someone giving you a Model-T Ford ;-)