For knobby 27" tires, check thrift shops that sell old bikes. You can sometimes find an old 27" wheeled bike with knobby tires for 75 dollars or so. Maybe replace the tires with nashbar cheap-o's and re-sell the bike on Craigslist. Now that I think about it, that's really a long-shot.
Used to be you could find 27" tires in department stores and sporting goods stores that sold bikes, but that's getting pretty rare now.