I've used an organization called Feral Friends. www.feralfriends.org They won't do the work for you but they will help you every step of the way. They were perfect and so very, very helpful. They have vets who work with them to do the spay/neutering. You have to be willing to put the time in to trap the cats, take them to the vet and pick them back up. Then you can release them back to your area.
It is helpful to have feral cats in the area to control rodents, etc. And, if they are "fixed", the colony doesn't continue to grow. I ended up with the kitten of one of the feral mommas who I was able to domesticate. I had no luck with his sister but at least she is living a life without giving birth over and over since I was able to trap both the kittens as well as the mother.
They don't require a fee but they are a non-profit and certainly are in need of donations to continue helping the way they do so donating a little to them would be kind.
Consider talking with them if you're still having a feral cat "problem".