As many of us do, I replace the heads of plastic figures with Hornet heads. Hornet has produced many new sets with great facial expressions and bald heads.

Does anyone make helmets (German and US) to fit those hatless heads?

Live Resin do WW2 german helmets in different sizes and finishes along with liners to go in them.

Never used them so can't comment on fit with regard to Hornet heads.

The German helmets from Live resin are very nice. Sovereign 2000 makes nice resin British helmets with camo netting (Mk II). Tank models makes/ made nice resin US helmets. The old Dragon 6808 "GI's Pusan perimeter Korea 1950", and 6054 "US tank crew NW Europe 1944", have good US helmets in plastic.

wow; that's a lotta helmets!

You'll have to file down the Hornet head, or cut it off just above the ears so a helmet will fit properly.

You can do what Biggles suggested which works great. Another way ,if you have a fig that is looking up and want to show that view,is to grind out the inside of the plastic helmet with a dremel and round grinding bit. I do it all the time. Just keep checking the fit so you don't end up with a helmet looking like it is sitting on a blanket on top of his head.
J