I ordered mine from Collage Video at www.collagevideo.com. I think that it cost about $7, but, once you add it to your shopping cart, you'll also notice that shipping is free. Search under Dyna Band and you'll find it.
They are usually called "resistance bands" or "Dynabands."
If you are looking for the kind Cathe uses, they are 6' long, medium tension. If you are looking for the 'large rubber band' type that Tracey Staehle uses, they have them at Perform Better and power-systems (see above for links).
Go to Amazon.com and search by Dynabands. The first listing is what Cathe uses and states that in the title. Make sure you get the 6 foot band. It's $7.95.
I got my band at Walmart. Mine is 5 feet long. I'm short, so it works ok for most everything.
The only thing I haven't had it work for is the bicep curls with band around the foot and held in the hands while lifting the barbell. I had all kinds of trouble trying to get that to work and just gave up and went a little heavier instead.