If you're looking for a free and effective fertilizer look no further than your local equestrian stables. Most are happy to have you come and take it. It is usually in a big pile (there is a funny joke in there somewhere) and all you have to do is shovel and bag it or shovel it into the back of your truck if you have one.
This is something my mom used in her garden all the time. I know she swore it was why her garden grew so well. The local horse ranchers in our area no longer give it away free until the fall. The way I see it, manure is a free fertilizer and I can always afford fee!
You can also check at the dairy or beef farms as well. They generally have a ton of extra manure they are trying to get rid of. Some of these farms will even deliver the manure to you. However, you will find you need to have these aged to guarantee you do not get the wrong bacteria of the E. Coli or other fecal bacteria in the ground.
Manure is wonderful fertilizer. The only thing is if you are organically gardening, the manure may have residue from any medications or hormones that were given to the animals.
Someone suggest this to me recently. It's a great idea and I am sure the stables would appreciate you making the work lighter. I had it in the back of my mind but have never drawn up the nerve to visit the stables.
My relatives actually swore by this stuff and they also got it for free, from local markets. They had the most beautiful garden and finally gave away the secret. If its free I wouldn't think twice to take advantage of the deal.
Horse manure is the best, due to their diet (unlike animals like beef raised for food and may have supplements). There are often "free" classifieds on Craigslist and other onlines like freecycle. One local rancher bags it here, calls it "garden candy" and mixes it with sawdust (I think that is to keep moisture in soil) and he has it year round for "free". For flowers, ornamentals and fruit trees and yes your garden, you can't get a better fertilizer.