I personally don't like Yosemite and much prefer Mavericks. Apple used to always charge a small fee to update but started making updates free after Mountain Lion. I am sure that if you go the official Apple update path they will insist on a full upgrade to Yosemite.
The last disc that was released for upgrade/fresh install was Snow Leopard 10.6 which was still available from Apple Store
That disc iso should be easily downloaded if you were so inclined.
10.8 Mountain lion was also $24.99 but I think it was no longer on a disc.
You can also download Mavericks and convert it to a bootable usb using diskmakerX. Lots of options available
I can help with some of these upgrades if you get stuck as I still have a few dmg files of the different OSX versions around the place
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