Which country are you in?
In the UK most farmers will booster the ewes a few weeks before lambing, and they then pass on passive immunity to their lambs which wears off by around 2 to 3 months old. If the farmer did not do this, then you have to be incredibly careful with rejected lambs and make sure they get as much (live) colostrum as possible. You can then administer a clostridial vaccine at 3 weeks old in the usual manner (double dose 4 weeks apart).
I would vaccinate the remaining lambs as soon as possible and if you can, feed them on preferably raw, but definitely non-homogenised goatsmilk (not cows) rather than milk powder.