if you just want a thin cardboard, i buy the big rectangular boards made for sheet cakes. you have to cut the squares out yourself. the cake drums in square michaels does not sell (at least at mine). so i get those at my local cake deco store too.
I look through the scrap lumber near the cutting center at Home Depot and Lowe's. I tell them the size square and they cut it for me. I think the most I've ever paid for a square board is .51$.
Christy
if you have time to order one i got an 18in square off of ebay. they also have them at www.cakesbysam.com. if not buy the rectangle cut them, stack like 3 or 4 then cover in fancy foil
all of the above....i've also used large square tiles from home depot. you can get a 12" for about a dollar . cheaper than a cake board. heavier, but sturdy.
What a wonderful idea! I love tile! Now I want to make a square cake.... 
Depending on the size, I usually by tiles from Lowes for my square cakes. Of course, that is only for up to 12". They will usually cur then to size for me as well. I then attach a non-skid backing to the bottom .
HTH
Janeil
I never thought about lowes cutting me a board, sounds perfect.
Thanks
Judy
i've done that too. at home depot, i buy a large piece (i forget maybe four ft by four ft.... for five dollars and have them cut several 12 to 24 inch squares. its very inexpensive. you can cover them in fondant, royal icing, "paint" them in chocolate, cover in foil, fabric, contact paper etc.
I then attach a non-skid backing to the bottom .
HTH
Janeil
thats a good idea. i've hot glued a smaller square of cardboard underneath so the tile doesnt scratch surfaces and so that there is a gap to reach underneath the cake to lift, but i like the idea of the foam non skid stuff. thanks.
I buy a whole sheet of Pinex board and cut my own. I use 16" and 14" the most as I like the board to be at least 2" larger than the cake on all sides. Cut up offcuts and glue them under the bottom so you can place your fingers under the board to lift up.
I use covered plywood boards and clear plastic Rubbermaid storage boxes to present cakes to my clients. I flip the boxes over so the lid becomes the base. And my clients always return them to me. 
My husband cuts 3/8" plywood for me for my cakes. He sands them to round the sharp edges, and then I cover the boards - top, edges and bottom - with white Contact plastic. Clear package tape on the back seals the edges of the Contact. Then I use wide heavy duty foil to cover the board for a cake, using masking tape on the bottom to secure the foil. My business card is on the bottom also, covered with clear tape. When my client returns the board, I just strip the foil off the board and wipe it clean with a mild cleanser. The boards have lasted for years, with an occasional recover with Contact. The 10x14 boards fit my 9x13 cakes, because the cakes always shrink a little. 14x20 boards fit my 12x18 cakes and oddly shaped character cakes. Both size boxes are about 5" high. I have about 20 of each size and soon will have 16x16 square boards to fit my new clear plastic 16x16 boxes I found for wedding cake tiers. The square boxes are 6 inches high.
Those boards and boxes, while a little expensive initially, have saved me a lot of money over the years. And the only trash created was the foil, so no cardboard from me in the landfills for my child and grandchildren to deal with in the future.
I hope this gives you some useful information. 
Where are we all from? I buy all my cake boards at what I call "the cake store". There are are few different pmes right around me so I would have to assume they would be everywhere. (but I am wrong often) I actually work at one of them as a part time job! Anyway, just go to yellowpages.com and look up "cake supplies"...see if anything comes up. They sell anything and everything you could possibly imagine for cake and candy making.
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