Oh, I am still looking around for one. I can't seem to find a slant back, they are all soft top or open. It's rather irritating.
The reason behind that, from what I've learned, is because the majority of civilian-owned Humvees out there for sale are from the late '90s USMC auctions in Barstow and Atlanta... 98% of those auctioned vehicles were 1986-1987 M998s... soft-tops. There were a few "rare" models released... some M1045s and M1044s from what I've seen; if you search here on the site, you'll find some old threads where folks bought some.
The problem seems to be that it's very rare to find a true M1026/44/45 with the weapon/ammo mounts in the rear and the armor plating... so folks who bought the few USMC ones wanted to restore them and hang on to them. Most of the slant-backs you see out there are 998s that are just partially mocked up to look like 1025s or 1026s... they don't actually have the armor plating or weapon/ammo mounts inside.
There are still authentic M1025/26/44/45 series trucks that come up for sale if you get lucky... I got a 1991 M1026 last year that was cannibalized by it's Army Reserve unit, then sold off as a rolling chassis. It had to be rebuilt, but the frame and chassis was original and still had the armor plating and weapon/ammo mounts.
Or your other option is to buy a 998 and mock it up... if you're just wanting the 1025 look and don't care what the data plate says.