I just bought a new 12" PB (which I love), and want to get a good sleeve for it. I like the sleeves that Brenthaven makes for the cases that Apple sells, but it appears from the Brenthaven web site can get them only with the backpack/bag that Apple sells. The one's Brenthaven sells separately are generic sizes. Has anyone every purchased these separately, and if so, how? Many thanks.