You could buy one with ios 7 and then make it look like ios 6 with cydia themes and tweaks, just a thought?
Joking aside, would iOS 6 even run on a 5s, with its 64-bit processor? iOS 7 must be coded to run on both 32-bit and 64-bit processors, but iOS 6...?
I don't know if there's a JB that helps you downgrade, or anyone selling a 5s with ios 6, haven't seen any for sale I may have to stick with 5 ios 6.13 for now.
No cydia on iOS 7, since it isn't jailbroken (yet).