Searching Craigslist and local classified ads would be your best bets. Farm and ranch stores such as Tractor Supply Store often have agriculture specific classifieds.
However, I would suggest looking for old hay bales instead of straw. Unless you are looking to do a strawbale-type of construction, IMO old hay will be superior to straw for almost any application. Straw often has lots of different residues of pesticides present. As long as the old hay has some herbaceous plants present like alfalfa or clover, you can be reasonably sure it probably isn't contaminated.
permaculture is largely about replacing oil with people. And one tiny ad: