Anyone know how I could possibly get my hands on the barcode scanning attachment that Apple employees use in the store, or if another similar product exists?
Contact Apple directly....as in their Cupertino location. When these things first came out, a lot of people inquired if they would be sold or not by Apple. Last I remembered, Apple was taking inquiries about them but not going as far as selling them just yet. Basically, you'd be put on a list of people waiting for this thing in the event they ever decided to sell them directly to small businesses/companies. You can probably ask your local Apple Store about these and they might do something for you but it all has to start at HQ in order for them to trickle down.
I realize that this may not be what you want, but there are several barcode scanner apps for the iPhone out there. RedLaser works great for me, and it's free.