I use Ivermec Cattle Drench, 1/2 ml per standard chicken...
Seems to work fine
It comes in a huge bottle though, but at least I have it. Not terribly expensive either.
Oh shoot, eta: I also bought a while ago from our local co-op a bag of water soluable wormer for horses, cattle, chickens, etc. I haven't used it yet, so I can't attest to its effectiveness, but just letting you know it exists. I can get the name for you in the morning if you like.
I use ivermectin paste wormer for horses, about a large-grain-of-rice sized dab per chicken. The couple times I've wormed the chickens, I just steal a little bit out of the end of the tube before worming the horses, then use that tube on the smallest horse