Dragon Gunner
Off the top of my head, for 28mm, there's Lead Adventure Miniatures, (see the great photos in the rulebook) Pig Iron Productions, and Anvil Industries. All great.
Modern Military miniatures: I'm a big fan of The Assault Group. Love their miniatures. Then you've got Empress Miniatures, Spectre Miniatures, (I'd like to get some of their GRU Russians) Footsore USA, Tiny Terrain, Eureka – - .
Again, I'm going with the Eastern European flavor. But really, you can slot in and re-purpose any figs you have and make your own Zone. Zona Alfa is a tool box, not a straitjacket.
I'm less familiar with smaller scales, but know that both Khurasan and Peter Pig have great 15mm Soviet-style figs and vehicles. Then there's 1/72nd stuff…
I'm sure other companies will come to mind, plus ones I've forgotten to mention. Forgive me for not having that info on tap.
Regarding vehicles, please understand ZA is a small scale skirmish game – combat between individual soldiers. Vehicles tend to be objectives or terrain.
That said, there are cheap Chinese diecasts on eBay. And a couple of sellers who specialize in 3D printed Soviet vehicles. A search will being them up.