Can I use the recovery partition or net installer to get a copy of Yosemite? Is that possible? I'd like to avoid a clean install. However, I need to upgrade a computer that would likely not run well with macOS Sierra. I need to upgrade a Macbook 2009 from Mountain Lion. My other MacBook has a version of Yosemite installed.
If you've installed Yosemite before, you'll still be able to download it from the App Store (given that you're using the same Apple ID as when you first downloaded Yosemite.)
Sign in to the same Apple ID on the MacBook running Mountain Lion, and go to the Purchased tab in App Store. You should then be able to find Yosemite and install it.
Yosemite I believe is a free download you should be able to go into the App Store and download the app there.