If you don't mind proxying, you can surely find some field guns and/or artillery pieces from other games. I have a pair of some kind of gauss cannon field guns for some game I never heard of, all in the wrong scale for BattleTech (I think they're 28mm the gunner figures that I didn't glue to the bases are almost as big as protomechs!) but... they look awesome as big bad 6mm scale "big bertha" style artillery guns.