You can also use the corrugated plastic that everyone is using to wrap their fabric around! Just make sure to round off the corners! There is a company that sells pieces of acrylic with the sole purpose of being a bag bottom, they are called Bag-E-Bottoms. Here is another option
http://www.tallpoppycraft.com/catalo...oducts_id=535.
I perosnally like using two pieces of plastic canvas cut to the size of the bottom, then tie the two together with a piece of yarn so they don't come apart. Instead of making a case for them to go in and can be removed I place them between the bottom of the bag and the lining before I sew the "turning hole" up. This way I don't have to fuss with it, it can still be washed like any other bag and the plastic will fall back in to place as you restructure the bag!
Oh and if nothing else call another non-local Michaels and see if they can order it for you! If they can then take that back to the other store as ammo to get them to order you more!