Ditto Fowgill, if they are under a year old, and you're only tagging for legal reasons, then don't tag them, just use a temporary mark.
If you tag, the buyer only has to tag again before slaughter, and this just causes confusion in the system.
If you're tagging for pedigree purposes, then you'd need to use "tattoo thing", unless you get permission from the BPA, and you really don't want to go the tagging route as it's double tagging and a real pain if they lose one, and the pigs can't be shown.
We use flag tags on our non-pedigree adult pigs for management purposes, and they don't tend to lose them.
If you're thinking of tattooing for the ones you're going to take to the abattoir, then you'd better check first, many abattoirs don't like tattoos as they're impossible to read most of the time. They like slapmarkers or tags.
If you don't plan to sell pedigree pigs, then temporary mark the ones you sell, and slapmark the ones for the abattoir, or use metal tags, or use plastic button tags, but check with your abattoir what their requirements are first before shelling out for kit.