"28mm vehicle weapon upgrades" Topic

Who makes vehicle upgrades like turrets, guns, and such that they sell separately from their vehicle kits.

As in, it does no good for me if they sell a turret and / or weapons for an entire kit, I need the kit to upgrade vehicles I already own.

Particularly I need turrets and weapons for generic insurgent technicals, like in the middle easy or eastern Europe, and Russian Gaz armored trucks, like this one pictured at the 2014 Russkie Victory Parade:
link

I'm sure someone sells this somewhere, but I'd prefer a US company or distributor due to shipping costs.

Thanks in advance for any leads!

There's this: auction

And some stuff on Shapeways:
link

Googling "28mm vehicle turrets" brought up some kits and STL files.