Tom, check out craigslist, I am sure I have seen manure for free.

Not sure if their website is up to date but Spirit Horse Center has/had some.

http://spirithorsecenterinc.com/news/#149

I don't have a source for you, and I don't know your level of knowledge, so these caveats may be unneeded, but for others wanting to use horse manure, I recommend reading up on the need to age the fresh stuff before using around plants you want to keep, and the possibility of pathogens (if using around edibles) or weed seeds, if not from a hot compost pile.