There's no guarantee on any route. You could get lucky on a short-haul that has some leftovers from the previous flight. I've also been on quite a few long-ish flights that didn't have any. Anecdotally, the outbounds from SIN have the greatest chance of having them. If the outbound crew were feeling charitable, the return to SIN might not have any left.

TLDR: it's all down to luck.