The FDA pulled most meds for chickens off the market about 2 years ago.
Sidenote: There is disagreement in the backyard hobbyist community. Many like to use DE and herbals to stay "natural." I don't personally prefer Diatomaceous Earth (DE) as it is bad for lungs (both human and chicken) and it does little to eradicate a build up. At best it can help prevent a mite/lice load, if they dust in it (breathing in the silica that causes lung damage) but it has to be dry to "cut up or dry out" the worms which is hardly effective in the gut. The "natural" herbal wormers are good to slow build up, as they help force the bird to expel the worms, but live, for the next bird to pick up. Once you've got signs of worms, there is a moderate to heavy load that I personally find can only be eradicated with meds. So unless you apply herbals regularly and rotate your fields appropriately, most of us on small holdings will be using meds at some point. I personally think they have their place.
The only thing on label for poultry currently approved by the FDA is fenbendazole in an extremely expensive poultry formula (Aquasol).
Or, you can use the same thing in a goat wormer under the brand name of Safeguard. Google that in the BYC search for treatment protocol. (I can't remember the dosage, but I think you put it in the water for 3 days straight. Generally repeat in 14 days). If you use the liquid goat wormer, place it in a gravity flow water feeder so that the med sinks to the bottom.
However, I see that Jeffers Pet Supply now carries in it pellet form to add to feed (it's like 1lb whole package for 313lbs of feed, so it ends up being a couple of scoops to a whole feed bucket...search BYC for exact dosage)
https://www.jefferspet.com/products/safe-guard-multi-species-dewormer?via=533884ad9fa2600f00000476/533884af9fa2600f00000499/533884b09fa2600f000004b1
If you don't mind being off-label (as in you aren't selling to the general public), you can use Ivermectin cattle pour on 1%, if you haven't been in an area of overuse. Place 5 drops at the base of the neck. It will remove round worms and lice/mites and scaly leg mites if a resistance hasn't been built up. Repeat in 10 days.
https://www.amazon.com/Durvet-Ivermectin-Pour-Dewormer-250mL/dp/B00JAL3AAW/ref=sr_1_2?crid=KD1C82DBTAXG&dchild=1&keywords=ivermectin+pour+on&qid=1588555467&sprefix=ivermectin+,aps,235&sr=8-2
My recommendations for what's left on the shelves for poultry.
LofMc