Have you? All of them I've encountered take a very standard 12V lead-acid battery - the same one that's inside consumer UPS units - and you can get replacements from a dozen brands on Amazon and other vendors.
I probably have to replace them every 2-3 years, though I imagine alkaline D-Cells would be far worse.
There's about 70 different sizes for these standard 12V lead-acid batteries though. And most of my consumer UPS units hide the battery behind a security screw and don't list the battery size in the docs. If the battery was removed and not replaced, it's harder than it should be to find a new one.
Tricky bit is getting a good charger if you're getting them for a device that doesn't charge its own batteries.
If you look at the voltage curves, Alkaline batteries start at higher voltage than NiMH but drop below NiMH voltage pretty quickly, so unless you have a device that stops working when your alkaline batteries drop below 50% charge or so at 1.2V, there's no reason to avoid NiMH.
And I thought NiMH was better at current draw than alkaline.