Where to buy MDF board?

Thinking about building a rack (as I have not been able to find any in the $500 range) and don't know where you can buy MDF board. Any suggestions would be appreciated as well as suggestions for any other kind of material, thanks, Richard
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.
Check out the Sanus Euro racks, i have a 4 shelfer and love it, strongest racks in their price range.($200. from Audio Advisor)

Here is the Critical Mass website.

Here is a review of them from Positive Feedback. They are nice but expensive.

I would search the archives here since many A'gon members have built their own racks and have shared their experiences.
Sorry if I am sounding rough but in my opinion a person who does not know a source to buy MDF is NOT a do it yourselfer and therefore ,don't try building one.It will cost you hours of work and probably it won't look the bussiness either,Just buy from a rack company and save your self the hassle.I did bulind my own aluminum rack but I got most of the commercialy avalaible tools in my garage plus I am in the construction work for years.Still it was a challenge and did hit my thumb hard and was sore for weeks.How?
A little scary but while cutting 1/2 thick aluminum on my chop saw ,I forced the 10 inch blade a little harder and with no lubricant,and the cut off piece went flying ,hiting my thumb.The blade at 90$ was warped and thrown away after only few cuts.
So my point is if you are not comfortable with power tools just let go and buy ready made .