Where to buy MDF board?

Rspark, Lowe's carries MDF. Or you can get a Sanus Euro rack (MDF). However, in your price range you can get a much higher quality rack. I'm thinking of getting a Critical Mass rack from Rainwater Audio. It was used in some high-end rooms at T.H.E. Show in LA this year.

Did not realize MDF was so easily obtained. Thanks for your suggestions about racks I might consider as I was not certain building one is the way I want to go. Any more suggestions about material/racks would be appreciated,Rspark

Home Depot sells 3/4 inch MDF in a size that works perfect when they cut it in thirds. I don't recall the dimensions exactly but I built a nice, inexpensive rack for my daughter's dorm room. One piece of MDF, four 36 inch sections of all thread, some nuts, washers, paint and you're ready to go. Way less than $50.

look at the VTI racks at racksandstands.com

Less than $500 and look great!

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.