You can use Safeguard Liquid Goat wormer or Safeguard horse paste, those are usually available locally. Or Valbazen, that is labeled for cattle and usually has to be ordered online. If you have birds that are molting then Valbazen is a better choice, Safeguard
may cause feather growth issues. Many have used Safeguard during molt with no issues, but some say that they have seen feather issues with it if used during molt, so it's possible.
Both are dosed orally, so you will need an oral syringe. It's not difficult to do.
Safeguard dose (for either form) is .25 ml per pound of body weight, dose 5 days in a row. Valbazen dose is .5 ml for a standard sized bird, two doses 10 days apart. ( Actual math for Valbazen dose is "weight in lbs / 2.2 X 20 mg /113.6" )
An inexpensive digital kitchen scale works well to get weights.
Lock them in the coop at dark, go out before first light in the morning and take them off the roosts and dose them one at a time (I would worm them all), put them in the run after, when the coop is empty you've done them all. They are usually calmer in the dark, for any that don't like to be held you can wrap them in a towel like a burrito, to hold their wings.
Pull down on the wattles, put the medication in the beak, more towards the front no more than .5 ml at a time, and let them swallow. Repeat until the whole dose is given. Don't squirt it forcefully, you don't want them to aspirate the medication. If using one of the liquids, shake it up well before drawing the dose, it settles out. You can get oral syringes at most
tractor supply stores and many feed stores down to about 3 ml size. Most pharmacies carry oral syringes, just ask, they will probably have 1 ml size. You can also buy them on
amazon, I buy them in bulk.