Centre lock rings came with the shimano rotors I ordered from CRC recently, if that helps.

2 Sets of centre lock wheels that I have bought (DT Swiss & Shimano) came with the lock rings- the front was a 20mm hub so needs a different lock ring to the ones supplied with the shimano rotors, it fits with a HTII BB tool rather than a cassette locking tool.

Shimano 15mm & 20mm hubs & wheels come with the oversize lockrings, rotors all come with lockrings, qr hubs just come with rubber covers, can't remember if shimano factory qr wheelsets come with lockrings or not

superstar do lock rings for about 4 quid

So if I'm selling wheels and rotors separately the lock rings should go with the hubs?

if your hubs are 9mm then lock rings would go with rotors

But if your hubs are any larger than 9 mm its a special lock rock which would have come with the hub - as catfishsalesco said the lock rings that come with hubs are tightened using a bottom bracket spanner, where as 9mm hubs lock rings are tightened using a cassette tool