Frontline spray can only be issued by a vet. You need the small spray (100mL), not the larger one with the big blue nozzle (250mL). It's not licensed for chickens but is extremely effective. It's the best treatment for Northern Fowl Mite-if you are ever unlucky enough to get them, dosed at one spray (0.5mL) per Kg bodyweight.
Frontline 'spot-on' can be got from the pet shop, but is far too strong for a chicken. I know someone who had the vet use it on her flock of Orpingtons and two died.