SafeGuard or fenbendazole is very safe, and it won’t cause sour crop or liver issues. A large worm load can sometimes cause crop or intestinal blockages, but fenbendazole takes care of them gradually over a period of days so as not to cause a sudden dying off of worms which could lead to problems. Valbazen or albendazole is another excellent wormer for chickens, and it is given once (0.08 ml per pound) and again in 10 days for most chicken worms except tapeworms.
TSC sells both SafeGuard liquid goat wormer and the equine paste which are 10% fenbendazole and the same dosage, and Valbazen is found in some feed stores, but more commonly online.