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.

MDF has Formaldehyde in it as well, in fact it is just another form of particle board, the particles are much finer and uniform that the cheap stuff that Ikea uses for most of their furniture. This fine grade of wood waste particle allows MDF to have a "uniform tear" thus you get a great edge when you saw it. I recommend using a carbide saw blade if you do cut it because the resins that hold it together will dull most other saw blades. Zaikesman has the right idea, laminating a "Formica" type product to MDF will give it strength beyond all get out, however you can't put a nice paint finish on it after you laminate it.