Hi and welcome!
If it’s armoured glass, it’s likely to be a specialist purchase and not something you’ll get off the shelf from your local Landy specialist or at the likes of autoglass. Try some ex-mod vehicle suppliers or see if you can contact the company who did the conversion. I suspect like-for-like glass, if you can buy it as a private individual buyer, won’t be cheap though!
Let us know how you get on! And don’t forget some pics of the Landy!
We’d usually recommend security being beefed up but on an armoured Landy, you might be okay on this one!