If your used to the XIM on the PS3 then please do yourself a favor and get the XIM 2 for the Xbox. XIM 1 on the xbox is great and all but it requires the use of an xfps which is going to add a horrible lag. The reasons for this are documented and extensive so I wont go into it right now. Just look around and see the responses for XIM 2.
Here should be the only place to buy one. We make them and we sell them. There is a secondary E-bay store as well thats sole purpose seems to be an intervention into people buying an xfps.
Look around, read a few responses, and make an educated decision. Personally, I'd get a XIM 2 for the 360 and a X-battle adapter to use the XIM 2 on the PS3 too.