I'll second the votes for B&H and Freestyle. B&H almost always has in stock what they carry, and Freestyle carries an even wider variety of b/w 5x7 film. There are also a number of smaller vendors who are committed to supporting large format photography and who regularly participate in the Ilford annual custom LF buy - Glazer's in Seattle, View Camera Store in Arizona, Midwest Photo in Ohio, and Hunt's in New England (several locations).