My abattoir were happy to take either tagging or slap marking. We decided to do the slap mark and bought ours from Supplies for smallholders, although you can get them elsewhere.
Unsure of how to do it we didn't want to leave it until the last minute, in case it went wrong or we chickened out, so slap marked a couple of days before they went. If you do it while they are feeding they hardly flinch.
It didn't seem to show up much through the hair so we took the marking kit to the abattoir with us to be on the safe side. We needn't have worried. When we got the meat back the slap marks were clearly showing on each shoulder.