I always considered Australian lamb to be a pretty safe bet as it is free ranged and grass fed. Bit hard to lot feed those bouncy little lambs but here's something to consider
Lambs are inoculated at 2 months with 6 in 1 vaccine, Scabiguard, B12 & Vit A, D & E, and use Click to prevent fly strike which is an IGR - Insect Growth Regulator.
At weaning (4mths) another dose of 6 in 1 vaccine plus B12 & Vit A, D & E + Click again.
For any lice an organophosphate (Eureka Gold )is applied and for worms, a drench with Lavamisol for internal parasites. This is standard practice
Trying to find someone who doesn't treat with chemicals is almost impossible – unless you’re buying organic lamb