I bought a new, unopened copy of Leopard second hand for $80 and, yes, the retail version can be installed on both platforms. Initially I was concerned I may miss out on 90 days of support because I did not buy it straight from Apple.
To my surprise, when I called Apple support about two weeks after I installed Leopard, they informed me I indeed have 90 days of support. I hope this person was right and not just well meaning.
I found my Leopard on Craig's List. If you find any sort of software for sale I recommend only purchasing new, unwrapped software just to be safe. There are a lot of people selling things they claim have never been installed when actually they have.