GPRS/GSM Shield

Hello,
I am working on a project to activate some relays using SMS messages sent from mobile, i have looked around for a GPRS shield and I have found some at third party stores but I couldn't find any in the arduino store. Did anybody have an experience with those shields? are they reliable? Where can I buy them?

It would be great if the shield came with all its accessories including antenna and so on.

If anyone could add an extra tip for making this project working would be great also knowing that i do not have a problem with driving the relays and I have already made the schematic for driving the relays with a digital arduino pin, all what is needed is just a shield to be able to read "RELAY1ON" and then the arduino puts 5V on a chosen pin.

Thanks alot.