Cider vinegar can be bought from horsey supply places and dosed at about 25mls per litre of water. Probably not best to use it all the time say 2 or 3 weeks out of every 4. Crushed garlic can also be used in the water as that is a mild antibiotic.
The powder referred to will probably be Diatom but this is best applied in a slurry. Enter Diatom slurry in the search box at the top of the forum page and you will get some good threads on the subject started by Hillfooter. Any dust from powders can cause the birds respiratory problems and should be avoided.
The best wormer to use is Flubenvet and although lots of people seem to have problems with the mixing quantities it is pretty simple if you read the instructions.
For lice Frontline for cats / kittens is good but not licensed to be used on hens although loads of people use it. Easy to use if you got just a few hens but might be a bit tricky if you have a few hundred
Use the search function on any of the above as there are loads of threads containing good info.
To buy the products it probably easier going online and searching for the cheapest. The larger amount of Diatom you buy the cheaper it gets and take note of whether VAT is included and also whether carriage charges are. Some places aren't as cheap as what they appear to be
If you do buy online it might also be worth buying some disinfectant like Poultry Shield for cleaning your coop.