Hey man, I'm in the same boat (although I guess I'm not quite ready to put my computer in my car yet, or even close for that matter). I emailed the mp3car store about a month ago and they said it would be a couple weeks, but they would be in stock.
There is another source for them http://www.mechatroniks.com/wrx/ordering.html (I suspect mp3car just orders them from this guy anyway). When I'm finished setting up my computer if the store is still out of stock I think I'm just going to order from that guy.
I also emailed the guy from mechatroniks.com and he says that he is still making them.