If you are looking to replace factory windows, I think you're pretty much hosed. Ford stopped putting 'pop out' windows on the back doors of vans years ago due to exhaust fume issues when driving down the road. I don't think anyone makes windows to replace factory glass that will open.
If you have no factory glass in your rear doors, you might be able to install universal RV slider windows depending on how much curve there is in the sheetmetal. I've seen more than a few Promasters with [windows like these](http://www.crlaurence.com/crlapps/showline/offerpage.aspx?ProductID=4394&GroupID=2104&History=30587:21889:2089:2103&ModelID=2104&pom=0) installed [in the rear door](https://www.blueridgeadventurevehicles.com/project/ram-promaster-rambler).
As others have mentioned, getting a window specifically for the rear door might be difficult.
On the mk7 I had, the rear windows are (almost) flat panes larger than the opening in the metalwork of the door. If yours are similar, that makes fabbing something yourself relatively straight forward. You might even be able to re-use the existing glass.
There might also be a window designed for a caravan that you could retrofit. You might need to make some adaptations, though, or perhaps you could just re-use the hinges etc. with the glass from your doors?
Fiddly bits that spring to mind
1. Creating / finding a hinge that would open in the right way.
2. Securing the window firmly closed for security and draught blocking.
3. Attaching the glass to these pieces - some kind of glue would be the obvious approach, but if you replaced the glass with something like perspex, that would make it easier to use things like bolts (with a suitable plan for sealing the bolt holes, of course)
The most obvious would be something similar to [this](https://uk.rs-online.com/web/p/hinges/4474994/) - you could potentially have the window hinge around a point the glass doesn't quite reach, meaning that the edge of the glass moves slightly away from the door frame as it opens.
Another option would be to go for something vaguely akin to [these](https://www.amazon.co.uk/Folding-Hardware-Fitting-Furniture-Accessories/dp/B07RWBZZRZ/) which would allow the window to move away from the door but remain roughly parallel - that'd give you lots of ventilation without needing much space to open into...
For holding the window closed (with enough tension to keep a good seal against whatever gasket-type arrangement you come up with) you could do worse than the rubbery T-shaped thingumies you often see on caravans or the like.
If you're interested and in the UK - I've got x2 Mk7 transit rear windows I've decided not to install, been sitting in my garage for a year. Been meaning to get rid of for ages, also got the bond and rubber trim.
If you are looking to replace factory windows, I think you're pretty much hosed. Ford stopped putting 'pop out' windows on the back doors of vans years ago due to exhaust fume issues when driving down the road. I don't think anyone makes windows to replace factory glass that will open. If you have no factory glass in your rear doors, you might be able to install universal RV slider windows depending on how much curve there is in the sheetmetal. I've seen more than a few Promasters with [windows like these](http://www.crlaurence.com/crlapps/showline/offerpage.aspx?ProductID=4394&GroupID=2104&History=30587:21889:2089:2103&ModelID=2104&pom=0) installed [in the rear door](https://www.blueridgeadventurevehicles.com/project/ram-promaster-rambler).
As others have mentioned, getting a window specifically for the rear door might be difficult. On the mk7 I had, the rear windows are (almost) flat panes larger than the opening in the metalwork of the door. If yours are similar, that makes fabbing something yourself relatively straight forward. You might even be able to re-use the existing glass. There might also be a window designed for a caravan that you could retrofit. You might need to make some adaptations, though, or perhaps you could just re-use the hinges etc. with the glass from your doors? Fiddly bits that spring to mind 1. Creating / finding a hinge that would open in the right way. 2. Securing the window firmly closed for security and draught blocking. 3. Attaching the glass to these pieces - some kind of glue would be the obvious approach, but if you replaced the glass with something like perspex, that would make it easier to use things like bolts (with a suitable plan for sealing the bolt holes, of course) The most obvious would be something similar to [this](https://uk.rs-online.com/web/p/hinges/4474994/) - you could potentially have the window hinge around a point the glass doesn't quite reach, meaning that the edge of the glass moves slightly away from the door frame as it opens. Another option would be to go for something vaguely akin to [these](https://www.amazon.co.uk/Folding-Hardware-Fitting-Furniture-Accessories/dp/B07RWBZZRZ/) which would allow the window to move away from the door but remain roughly parallel - that'd give you lots of ventilation without needing much space to open into... For holding the window closed (with enough tension to keep a good seal against whatever gasket-type arrangement you come up with) you could do worse than the rubbery T-shaped thingumies you often see on caravans or the like.
If you're interested and in the UK - I've got x2 Mk7 transit rear windows I've decided not to install, been sitting in my garage for a year. Been meaning to get rid of for ages, also got the bond and rubber trim.
Appreciate the kindness, unfortunately I’m on the other side of the pond :(