Can't reinstall OS for iMac 9,1 (2009)

Hi

Bought a new iMac so was cleaning out old iMac 20" to pass on.

Did all the disk utility clean of hard drive & back-up disc, but have now entered the 'El Capitan doom-loop' for installing an operating system.

How can I get an operating system installed so that I can pass the machine to another user?

Once upon a time I had a Snow Leopard set of installation discs, but I can't find these. Apple Store now only offers downloads of Lion.

How can I make this old machine usable again?

Thanks