presumably we're just talking about soft or hard plastic models here.
There are a number of alternatives in metal, try SHQ Miniatures or Early War Miniatures, with crews.
There's also Butlers Printed Models in 3d printing but no crews.
They do 105mm howitzers, 37mm and 57mm a/t guns and 75mm pack howitzers, all can be printed in various scales.
I can recommend all these and hope this helps!
Stay safe folks!
a few more thoughts from my old memory.
Hasegawa do some 1/72 scale kits, which included jeep and 37mm a/t gun, and also
a 155mm Long Tom gun, also a High Speed artillery tractor though I don't know if they're still available.
Italeri used to do a 155mm sp gun in 1/72 scale.
There are also the old 1/76 scale Matchbox kits of M16 AA half tracks, a 155mm sp gun and the Priest.
In addition Plastic Soldier Company do a Loyd Carrier with 6 pounder a/t gun with parts for the US 57mm version.
Hope these thoughts help - when I was young I had the problem of finding US guns but they're out there in one form or another!