You might be able to find this part on eBay or elsewhere.

There have been at least three Y-adapter cables since 2007. They are in order of release.

1. 61120440645
2. 61120440796
3. 61120440812

#1 above is the oldest one and does not work with all iPhones/iPods. I have #2 and not had a problem with any of the many iPods and iPhones that I've connected to it (including the newest touch's and the iPhone).

The part number is shown, as embossed writing on the cable, as the last 7 digits. Mine is "0 44 0796".