You are looking for the Dalianjj's trailer. The box looks similar to the Justitoys one. They sold on ebay for about $120 USD for awhile, but they have long since been sold out. I'm scared to think about what they go for now.
Here are some photos
http://www.tfw2005.com/transformers-...Prime+Trailer/
Here are some reviews
http://www.tfw2005.com/boards/transf...-promised.html
**edit**
Oh I see that you have one. They also sold upgrade panels for the interior (cool panel lines to replace stickers) and exterior (molded stripe to replace sticker) of that trailer to make it look a little nicer.
