Someone on twitter was asking me where to get straw bales; you can look on craigslist, in your local ag newsletter (can sometimes be found at the front of local grocers or restaurants), and usually plant nurseries and farm stores carry them, as well. 

Your most inexpensive source will be to find a local farmer.  Nurseries around here charge as much as seven bucks per bale!  I pay three.

The great thing about straw bale gardening and mini-greenhouses is that you can make them as big as you want.  Just add more bales and more windows!