Horse manure is better than cow - but it needs to be well composted before use or you'll have a pile of weeds. I'm afraid that I'm not familiar with sources in NOVA, but I did a search for "horse boarding, Northern Virginia" and a number of hits returned. Northern VA is big fox hunting country, so there should be plenty of stables in your area.
I don't know how far you are from Gainsville, VA., but there is a huge Alpaca farm there. Alpaca manure is primo stuff....
Good luck!

Meg :)

Here is a link that might be useful: Alpacas in NOVA

Saw this on craigslist - in Vienna. No idea who or where they are.

http://washingtondc.craigslist.org/nva/grd/584937972.html

http://washingtondc.craigslist.org/nva/grd/583124002.html

Here's another one. In Hamilton (a little west of Leesburg).

You can get composted horse manure at the National Park headquarters in DC at the intersection of Rock Creek Parkway and Military Road....bring your truck and they load it, or bring bags. And it's already composted!