I can't see how raising lambs for slaughter with a commercial breed without breeding them yourself can be worth it. As Worzel says, some are ready at 3-4 months - which is when they are weaned, so no time for a fattener. You could buy 'orphan' lambs - which might be genuine orphans so will not have had the best start in life, or could be triplets taken off the dam to let her raise just two. Then you would have to bottle feed (lamb milk replacer is fairly expensive). Rare breed primitives, such as Manx Loughtan, Shetland, Hebridean, etc, are usually reared to 16 months before slaughter - the reason for this is that they don't grow fast enough to be ready at the end of the Autumn, nor will they put on any size over the winter no matter what you feed them, but they will grow away very well once the spring grass comes in and will be ready by late August from April born lambs (from the previous year). So all you would need to get them to slaughter size is plenty of good grass and some hay for the winter, plus a mineral lick. If you go for this option, Shetlands are probably your best bet as they are far more numerous than the other primitives, so you would get meat lambs fairly cheaply and you would be able to find a breeder local to you fairly quickly - contacting the Shetland Sheep Society would be a good start (there is a separate society on Shetland itself) or find a local breeder of unregistered Shetlands and buy wethers. I've just checked where you are and there are several Manx breeders in your area - they have the advantage of being bigger than many rare breeds, but they get fat if you keep them on too long.
You shouldn't need to have anything to do with ear tagging as they will go in on the holding of birth.
When to buy - either at lambing time if you go for orphans, or at weaning (we wean at 4 months but others may wean earlier), and the actual date obviously depends on when the lambs are born.