@gtwilson I have links to an Apple website where El Capitan & Sierra can still be downloaded. Do you have a Snow Leopard DVD? The retail version of that was 10.6.3, predating the Mac App Store which arrived with 10.6.6 before topping out at 10.6.8.
Maybe you can get that DVD (still available to purchase for $19.99) and progressively apply Software Update until you get to Lion. Maybe. See https://www.apple.com/shop/product/MC573Z/A/mac-os-x-106-snow-leopard
I have disk images for the original release of Lion & for the Combo Updater to 10.7.5.
There were a few supplemental updates for Thunderbolt as well about that time.
// @matigo @streakmachine