Saddle keep tilting. Which spare to use to replace it?
First thing to do is double check the other side is put together right, namely that the toothy bits are engaging. 

If so, probably what's happening is the teeth that are supposed to hold the angle adjustment in place got rounded off from prior incidents of slipping.

These kinds of clamps come in one very commonly seen size, 22.2mm aka 7/8", and a couple weird ones that are mostly the domain of antique bikes. You might as well bring in the old one to match, or put a ruler across it to confirm it wants a 22.2mm post end (different from the seatpost size proper because usually pipe style ones taper at the clamp end). The rail mount part is universal among normal single rail saddles.

Most bike shops with any kind of family/casual customer base accumulate a big heavy box of new replacement clamps because they come with certain brands of  saddle, but most bikes don't actually need them because they have posts with integrated clamps. So from a shop like that they're pretty ubiquitous and cheap.