I expected joemike's response, but since I had already posted here I had to ask. (On the other hand, though, if push comes to shove, my El Cap will be transferrable when/if I upgrade to Sierra.)
I'd try calling Apple; they must still have El Cap available for users who haven't upgraded yet and whose Macs can't run Sierra.
I found
Download El Capitan.DMG...without Apple store, but I don't know if it's still a good link. (I've got no knowledge of macdrug.com, but the Google search responses I read were all positive...not that anything makes me particularly anxious to d/l OS X from a party other than Apple.)
Edit: Actually, I've always found it kind of amusing that we, here, are so fastidious about Apple's EULA while Apple does absolutely nothing about the bootleg OS X installers that litter eBay (and perhaps other venues).