You can buy it at Wal-Mart--look for Heat N Bond LITE--it's the purple writing. You can sew thru it--the ULTRA you can't get your machine's needle through--it gums up.
Some fabric stores sell Wonder Under by the yard too. I used to be able to buy it off the bolt at Joanns. Not sure if they still sell it that way but I'd be surprised if they did not.
hth,