Double question fly problem and when first treat for worms?

Hi guys I've been raedinf about flubenvet or something like that for worms.  When should I first give my girls a dose they are 19 weeks old on Saturday. do I get this from th vet or the poultry shop I got my hens from? Also I seem to be atracting flies the hens walk over a stoned area and it's hard to wash all the poo away any sugestions is detol safe to disenfect everywhere? or will that make them sick they are likely to be pecking where I plan on disinfecting. 

Thanks Mr magic toad  I'll Buy it on line then. any idea what age I should give it to them?  for the first time I mean