Basically a sharkport and x-port are the same thing except sharkport is the current U.S. version. ( I believe they are doing away with the name "sharkport" and calling them all "X-port")
Anyway, a sharkport is a package that comes with a USB cable, and two CDs. One CD is to install the needed program to your computer. The Second is a transfer software CD to put in the PS2, so it recognizes what to do with the info that is going to it and coming from it. You can put game saves on your PC, or cars individually or replays. Pretty much, anything that you can put on your memory card can be transfered to your PC. And then sent to other ppl by email or posted on this forum, if you want, you can swap your garage or replays with other GTer's. Pretty cool, huh?
Hope that made at least a little sense. If I left anything out, someone please add to what I've said, or correct me.
As for getting one right now, I think your out of luck unless you wanna pay $50.00 on E-Bay. I'd wait until they transfer to the new x-port label. Unless you're on the PAL(UK) version PS2. Then, I don't know where to get one.