Hello,
maybe you take a look at the HäT WW1 range. The gun crews no. 8157 - 8159, 8256 - 8260 have seated soldiers, the both German Field Wagons 8260 & 8261 are useable. 8109 is WW1 German Artillery with limber. HaT Set 8126 is German WW2 motorcycle with sidecar, included sitting soldiers in leather coats, riding shotgun. The HäT tank riders may be fine, too.
Look at them at "Plastic Soldiers Review".
Hermann