I chose the Waterfield Dash sleeve (#37-12" MacBook) because I was looking for a minimal, padded sleeve that was durable and professional looking. I got it on sale, for around $50, I believe. It provides a snug fit, and inside the mesh side pocket, I can carry the charging brick/cords and/or my X3 if I'm just carrying my LapDock around. Otherwise, I can slide the sleeve inside my larger Timbuk2 messenger bag.
https://www.sfbags.com/products/dash...ok-ipad-sleeve
If you are looking for something roomier, with more carrying capacity, you could also look at this Case Logic bag, almost a "mini-computer bag". I had considered it, but it was really larger than what I was looking for.
https://www.amazon.com/gp/product/B0...pUvbUpU3696477
I had also considered these two sleeves, and they would have been OK, but not quite the quality construction of the Waterfield, but granted, they are a fraction of the cost.
https://www.amazon.com/gp/product/B0...pUvbUpU3696477 https://www.amazon.com/gp/product/B0...pUvbUpU3696477
Good luck, and enjoy your LapDock!