The obvious place for commercial ones would be Spectre Miniatures, who sell a load of weapons to go with their own technicals (when they have stock). The usual price is only a few pounds, which isn't much if you're not spending much on the original vehicles themselves.
https://www.spectreminiatures.com/collections/vehiclesEureka also do loose weapons I believe. I did buy a DShK from them one time, though I think they're more expensive than Spectre. I would say there's others companies as well, but quality wise you're seeing diminishing returns for similar prices.
For spare turrets I'm not sure what's available in that scale. Die Waffenkammer make 1/56 turrets available, as do Company B, but they're going to be too small. Empress have Imprint's stuff, though whether they'd be willing to sell individual parts if you email them, I'm not sure. Kings Hobbies and Games do have loose 1/50th turrets for modern vehicles, but they may be *too* modern to be showing up on a technical (though if you PM Tim on Facebook he may have 3D prints of other stuff available; the prices may be too high for what you're looking for however).
Edit: ah, and there's this site doing 1/43rd scale resin vehicles too. They're moving into doing technical, so may be selling the weapons separate (its early days) if you contact them.
antenocitisworkshop.com/antenocitis/aw-survival/