I have previously purchased it at Hobby Lobby when it was 1/2 off.  They do not do this anymore :(  I was trying to find it on their site but can't find it on line and the closest one to me is 1 hour away.  Was wondering if there is a cheaper place to purchase.  The last ones that I bought were $2.49 for 20" x 30" x 3/16.

dollar tree
 

Where about in MI are you?

I go to Foam Factory in Chesterfield. I tell them I need foam boards and they usually have pretty large scraps around that they don't charge too much for. I got some 20x20 squares for about $0.80 each. They'll make specific sizes for you too, but it's a little pricey.

I get mine at Global Sugar, they are pricey in my opinion, but I just work the cost into my final price.  I like them though because they are 1/2 inch thick and very sturdy.

I live in the tip of the thumb.  Quite a drive to get anywhere. :(

I find the foam core at Dollar Tree to be woefully inadequate.  The paper surface is "odd" and it's very flimsy in comparison to the quality Elmer's board available at Michael's/Joann/Hobby Lobby.

The craft stores often have quarterly sales on foam core with individual boards ranging from $1-$1.50.  I stock up then or use coupons whenever I can to get them cheaper.

I also coat all of my foam core with a thin layer of melted, edible food grade soy wax--

Golden Blends 415

available from Ebay: http://www.ebay.com/itm/130491955139?ssPageName=STRK:MEWAX:IT&_trksid=p3984.m1438.l2649

I also get mine at the Dollar Tree. What I do is cut it to the size I want, sandwich in cardboard from boxes (a lot of store will give you empty cartons they get stock in if you ask) I've also cut to size, and on the bottom another piece of foam core...that way the bottom is nice and white. This makes a nice sturdy base. Then I cover it all with foil or freezer paper and ribbon around the base. I might add more cardboard in the middle depending on the weight of the cake. If it's a wedding cake, I usually use wood.

Thanks everyone for your replies.  I'll be checking out dollar tree the next time I'm near one. :)