Great War Miniatures make Tommies in gas masks.
Germans too, but I don't have a pic of them. Might want to check out Renegade too, I seem to remember them doing some. Don't know of any separate heads, but Woodbine have started doing separate WWI heads and who knows, they may do some gas mask ones at some point.
For the second part of the question you may want to get some of the HLBSC British artillerymen before they stop selling them (end of March, I think). Not useful as drivers, but they fit the machine operators remit.
I've just bought a load of these (four packs I think) to use as artillerists, but also armoured car and tank crews in case they have to bail out during a game. Most of them can be used simply as "troops doing nothing" which are always useful. I plan to headswap a few of them for various other jobs. Third from the right can also be used for a 28mm version of Edvard Munch's
The Scream.
But as I said, be quick. They are stopping casting them soon.