Plumbing fittings are highly standardised. If you’re unsure, take a photo of your pipe or the sink waste fitting. Head to the (often near the pipe cutters) and ask for: “A rubber test plug to fit [32mm or 40mm] PVC pipe.” They’ll point you to the exact size.
If you just need to block a hole and don't care about aesthetics, this is the budget-friendly choice. bunnings plugs sink