My personal oppinion is to start with a pair of jills, so they can be from the same litter, try finding a pair from working stock, but again its hit and miss, there are that many ferrets breeding now that have been pets for years and years will probably have hardly any intrest in working!, best bet is to try get some kits of here!.
Some say wait over a year to work them, i put mine in around 6 - 8 months and let them follow normaly their mother or an older ferret down.
Atb.