Scheltrum - great interwar vehicle range,
Copplestone - good range of 30s armour,
Empress - modest SCW range
brigade games - couple of good vehicles trucks, in their 30s pulp range
Company B - (also stocked by brigade games) mainly WWII, but a small number of 30s vehicles which were will around when war broke out.
JFTM Enterprises: same as comp B
Warlord - same as comp B, I do particularly like their loyd-carden carrier
Force-of-arms - great SCW range, but the company is on hold at the moment - however you can still get most of their range through great hall games
Under-the-bed Miniatures stock some vehicles as part of their VBCW range (which I believe are revier miniatures)
Reverscio (?) Miniatures: Do some interwar trucks an cars, but very old school sculpting
1st Corps: do some interwar motor bikes.