Where to buy MDF board?

Be very carful when cutting MDF, most brands of it give off toxic gas when sawed. You should always wear a filtering breathing mask, (not the cheap paper cone type) when cutting MDF. There is some MDF available that doesn't give off toxic fumes but it's much more expensive and the guys at the big box hardware stores will look at you like you just beamed in from the plane Zorbon when you ask them about which type MDF they sell.
I would take some of the earlier suggestions and look for either a used rack or one of the above before I started messing with MDF, not only for the toxic fume reason but because it is very fragile, unless you use a hardwood or metal for the frame, it has a very low shear weight component. MDF is really only good for shelving.

home depot,lowes or any other lumber yard carries mdf as a staple.

One suggestion was a Critical Mass rack from Rainwater audio...I searched the web and could not find a site. Can anyone help me with a way to a site (or any other rack suggestions) Thanks, Rspark

I think Mjw55 is thinking about partical board which has formaldyhide in it. But it is a good idea to use a filter when cutting any wood. Most people don't realize birch is poison.

I bought some from home depo, and they cut it for me. No charge.