Cable to connect a guitar to my iPad

Hi,

I'm hoping that someone out there will have seen this too.
I read somewhere that there is a cable to use with Garage band and other Audio apps that plugs into the 30pin connector.
This then leaves the headphone socket free, and I think gives you a couple of ports to plug into.
I have tried the irig and the iShread cables but seem to get a bit of lag on them from time to time.
I had heard that this can be reduced with this other cable. But I can't seem to find what it is called and were to buy one.

Can you help????

Cheers,

Pete