Where from to download older OSX version (10.9)?

Not doable. Not only isn't Mavericks available any longer, computers normally cannot run with an OS preceding the one that shipped with the machine.

See these for details:

Computer-specific Mac OS X releases,

ICBM Builds, and

Don't install a version of Mac OS X earlier than what came with your Mac.

27" i7 iMac (Mid 2011) refurb, OS X Yo (10.10), Mavs, ML & SL, G4 450 MP w/10.5 & 9.2.2

You have to determine which one came with the machine. If the latter three, that's Yosemite, and the computer can't operate with anything less than that; most likely since the hardware's been changed.

If the latter three, that's Yosemite

No, the latter three are Mavericks.

Make a polite call to Apple support is what I would do. Explain what you need to achieve. It shouldn't cost you much time and/or effort to get a definitive answer.

For whatever reason, be prepared for them to say No.

Bear in mind that although OS X will run on hardware that isn't specified in the listed requirements, it can be problematic and is probably against the EULA.

Ah! You're correct. Those build numbers match the three OS 10.9 versions. Had a brain ****. Then, booting into Internet recovery mode and reinstalling the OS should put what came with the machine back on. W/o a recovery HD, then try gumsie's suggestion.

Let us know how it turns out.