We've purchased some frames, but long-fibered sphagnum is either not to be found, or found in tiny (expensive) bags in our area. Are there any legitimate, workable alternatives to stuffing a wire frame with sphagnum?

My brainstorms were mostly... well, stupid. One notion I had was lining the frame with handfuls of straw, then dumping potting mix into place inside. It seems the weight of the mix would hold the straw in place. Then again, I imagine the more experienced members of this forum would say the water retention capacity of my straw/dirt mix would pale in comparison to sphagnum.

Any other ideas or time-tested methods?

--RLS3