I just got this one (in another color) from Amazon for $19.99:
MoKo Slim-Fit Multi-angle Folio Cover Case for Barnes & Noble Nook HD+ 9-inch tablet (Full HD Plus), BLACK (with Smart Cover Auto Wake/Sleep Feature)-Lifetime Warranty
The multi-angle bit is mainly for landscape mode - it's not really designed for the Nook to be propped up in portrait mode. But when the cover is folded back in portrait mode there is hand strap to hold it more securely. The cover fits snugly with a soft hooked clasp, and there is also a vertical elastic band attached that can also be hooked around the Nook to make sure it stays closed if it's in a handbag or whatever and to keep the cover against the back when folded open. The reason why I got this one is because it has vent holes where the speaker is, so it doesn't block the sound like the cover I have for my Nook HD does.