The only hardcase batteries I know are the stock Dynamite (well now Spektrum smart) packs, and I believe Gens Aces pack for the Mini is a hard pack. I currently use cheap urgenex ones off of Amazon. They are a soft pack and they are pretty good batteries.
Or you could 3d print a case, or buy a...
https://www.horizonhobby.com/product/7.4v-800mah-2s-50c-hardcase-lipo-battery--ec2/DYNB825H2.html
Never seen actual lipo hard cases sold separately than the battery. I would get those dynamite ones I linked above. I got 2 of them for my mini b. Work just fine. Just a little less run time than...