I would think you would have an easier time there (Portland) than I would here (Augusta). Be sure to call all the "suspects," and, especially if you are dealing with greenhouse/garden centers, be sure to ask about 4 and 8 cf bags. I got lucky when I stopped into Longfellows in Manchester, and was asked if I would be interested in an 8 cf bag when I asked about 4 cf ones. Ultimately I got 8 cf for around $30 (it was more than a year ago, I think it was $27 something). I suspect that some places I asked might have had the bigger bags, but either the employees wouldn't know to look them up or offer them to me, so I would be thorough, both in calling places and in asking what size containers of vermiculite they have at what prices.