Chole,
I don't really know anything about reenactments, but are you allowed to double strand it? You might be able to find something "chunky" and double strand it to get the 2.5 stitches per inch. Or heck, even triple strand something thinner.
Or, try this:
http://www.yarndex.com/yarn.cfm?yarn_id=597 if you click on the gauge size, it will search for other yarns with the same gauge. Many yarn shops carry Brown Sheep and should be able to get the burley for you (or you could find it online), or whatever else you might choose.
~amey