It depends on whether you want pre-medicated pellets, which you feed to them instead of their ordinary ones for 7 days, or whether you want flubenvet powder, which you add to their normal pellets before feeding for 7 days. Both have advantages and disadvantages - the premix is easy to use straight from the bag, but you're unlikely to use exactly the amount you buy, and any spare will be out of date six months later when they need doing again. The powder is easy to mix, just weigh out 1 kilo of pellets per hen and add the powder- it comes with a scoop so its easy to work out what is needed,( 2 scoops to 6 kilos of pellets.) You can mix the right amount for your number of hens with no waste, and the powder can be safely stored with a longer shelf date than the premix pellets. A pot does 20 doses, so if you had 10 hens it would do two treatments. You have to feed ONLY PELLETS for the 7 days, no treats, so each hen eats the right amount for her bodyweight, and doesn't fill up in extras.
For premixed pellets, Google 'Flubenvet premixed pellets', and for powder, Google 'Flubenvet wormer powder.' Or check on Ebay, prices vary quite a lot. Then work out what's cheapest at the moment, remembering to add in postage costs.