Where can I find an installer for OSX Yosemite?

Open your Safari browser and click on the link below.

How to get old versions of macOS – Apple Support

Go to Download macOS and click on OS X Yosemite 10.10

this downloads InstallMacOSX.dmg to your Downloads folder.

The next section can only be done on a mac that is capable of running Yosemite,

a mac that came preinstalled with an OS later than Yosemite will refuse to do the next bit.

When downloaded open to InstallMacOSX.pkg, double-click on

that and an installation window will open, this does not install Yosemite

but converts the InstallMacOSX.pkg to the Install OS X Yosemite.app which 

you will find in your Applications folder.

 (If the installation window asks which disk you want to install to, you must pick 

the disk that you are booted to at the time. Not any internal or external disk that 

you want to eventually install Yosemite on, that is for later.)

To start the installation of Yosemite double click on the Install OS X Yosemite.app.

The copy of the install app self deletes after installing Yosemite, so make sure you keep a copy of the InstallMacOSX.dmg

if you need it in future, or you could just make a copy of the Install OS X Yosemite.app prior to installing and moving it to

an external drive for safe keeping. It is also possible to create a bootable USB installer disk using the Install OS X Yosemite.app

in the Applications folder and the createinstallmedia command in the Terminal app. 

Read the instructions here,

How to create a bootable installer for macOS – Apple Support

sudo /Applications/Install\ OS\ X\ Yosemite.app/Contents/Resources/createinstallmedia --volume /Volumes/MyVolume --applicationpath /Applications/Install\ OS\ X\ Yosemite.app

How to get old versions of macOS - https://support.apple.com/HT211683

Note that some users have reported problems getting the links to actually show a page in the Mac App Store App when used with Firefox based browsers. Try using Safari instead.

Does the computer already have a partition for Yosemite? If so, just download the installer and install the OS to the partition while in HS.

Realize that Yosemite pre-dates APFS formatting which it is likely your High Sierra uses. When you are booted to Yosemite it won't see anything on an APFS partition. You will also have to make sure you format the Yosemite partition to HFS. A bunch of the formatting discussion depends upon what kind of drive (HDD/SSD) you are using for this. I just spent an entire week researching how best to set up a Mojave, High Sierra, El Capitan bootable setup with an internal HDD and external SSD.

ExFat is a newer variant on FAT which is used for basic cross-platform Mac-PC file exchange. If you buy a digital camera it will use FAT so the cards can be read by pretty much anything.

HFS applies to recent Macs prior to High Sierra back when hard rives were mostly rotational disk drives (HDD). APFS was introduced properly with High Sierra and optimizes use of solid state drives (SSD) drives. High Sierra uses it but will accept being installed on HDDs. Mojave will convert anything it is being installed on to APFS. Things just get a bit messy if you have a HDD and are trying to use it with Mojave because APFS on a HDD can cause extra drive activity. APFS also gets messy on dual boot systems since old OS versions up to and including El Capitan can't "see" an APFS drive.

The link I posted earlier has the information your need, from Apple itself. I just figured you could click on a link and read it.