There is a lot about this on other threads. With pigs, once you wean the litter the sow goes back to get in-pig again. With most breeds of sheep, they only lamb once a year, so there is no advantage to the farmer to wean them early for you to rear. Many breeds go for slaughter straight off their dams. This leaves the alternative of buying orphaned lambs - either genuinely orphaned because their dams have died, or lambs which would be bottle fed because they are the third of triplets. Reading through the posts you will see that there are many problems associated with rearing these lambs, and their viability depends very much on the source ie if they have been reared to the point where you buy them by someone who has made sure they have had adequate colostrum and good care, or if you get ones which have not had that beginning. It seems to be pretty much pot luck, unless you know someone you trust.
A possible alternative would be to take weaned lambs ie 4 months old, of a slower maturing breed, and raising them to slaughter weight. This could be anything from a few months to a whole year.