If you are referring to MDF (Medium Density Fiberboard) sheets, then you can get them in various thicknesses at any Building Center.
If you want 'Flat Sonotube', then I believe you are out of luck.
There is also a product called LDF (Light or Low Density Fiberboard) which they may have at the building center or they may need to order it. The main difference between MDF and LDF is weight (and some strength depending on the application).
"Compressed fiberboard" also includes what is more commonly called "hardboard" (in addition to MDF, as Paul mentioned). Hardboard is the stuff that pegboard is made from. Clear (non-perforated) hardboard is available at any building supply superstore.
Sonotubes are spirally wound, laminated fiberboard. If you're interested in this type of laminated fiberboard, a thinner version is commonly used to protect pallets of wood sheet goods at Home Depot or Lowes (they're called "slipsheets"). I got some from a Lowes once for free.
Daniel-- elaborate a little more if none of these products we've mentioned is what you're talking about.
I think I meant to say compressed fiberglass it's like the stuff you have up in the attic except it's compressed and not fluffy. I've seen it in tubes that you can wrap around pipes and have seen them used to make bass traps and I was wondering if there was any place I could get them in sheet form.
Nope, sorry, you'll have to make some calls to your local Building places. Can't see it being that expensive, I've seen some similar acoustic sheets (which I think is also compressed 'glass but wrapped in plastic) for about 4 buck for a 3x3 sheet, here: http://www.mcmaster.com/catalog/109/html/3237.html#.
Paul-- The slipsheets are usually pretty beat up. Also, they're a LOT less rigid than luan. They're like a thick cardboard.
I was experimenting with making a curved piece of base moulding. I laminated together about 8 pieces of this slipsheet material with regular yellow wood glue. I ended up not using it, as it didn't mill well. (A router frays the edges; a saw cuts cleanly though). As a flat sheet good though, you might be able to glue up about 4 thicknesses of it. After all that work though (and the cost of the glue), I think I'd rather just buy a sheet of luan!
/hijack on/ Thanks Mike, I may take a look at them anyways as I do Giant Scale RC stuff and am always looking to source 'free' wood for misc projects, even as scrap for fiberglass mould work. /off/ Good luck in your search Daniel