Why can't I find OS X Yosemite in the Mac App store? [duplicate]
Solution 1:
Unfortunately the OS X Yosemite installer is no longer available on the Mac App Store.
If you have a paid Apple Developer account you can download it from https://developer.apple.com/downloads/
Solution 2:
user163914 is correct. I would have commented on that reply but lack of rep.. sigh. I see Yosemite 10.10.3 build 14D131 (release date April 8, 2015) available at https://developer.apple.com/downloads/ There are two files (the actual Yosemite update and a combo update file. Both are over a gigabyte in size, be warned) but you only need one of them. Note that it is the only Yosemite install version that comes up in the search. Hope it helps though.
Apple dev accounts are free by the way, all you need to do is sign in with your Apple ID and accept the dev agreement.