There is a list of authorized resellers here: http://www.shieldzone.com/company/reseller/index.html
I emailed the invisible shield CEO to ask him if they could make just the back shield available. I don't feel that the front of the itouch is going to get very beat up, just want a protector for the mirror back.
I sent a similar email. Here is the response I got:
"In regard to your e-mail, even though we don't currently offer a shield for just the back of the iPod touch, you can order it through our Custom Order process on our website. If you put in the Custom Order notes specifically what you would like, a representative will contact you for the balance of the cost of the shield (since we already have it), and then we will get it shipped out for you."
My shield shipped on the 20th and I recieved it in the mail today. I am in Upstate NY