Crooked Dice uses slotta bases. Though the figures aren't yet available, their Kickstarter just added significantly to their pulp line. And while the majority of the catalog is outside of the 30s to 50s range, with a little bit of searching there are a number of figures that would fit right in.
Depending how far afield your adventures take you, TinMan Miniatures has some figures perfect for Lost World pulp, but with pegs for their sculpted bases rather than slots. I WOULD WARN YOU, however, that some TinMan minis have integral bases. You can try looking at the higher res pictures in the catalog, but even then it's not always apparent. Beautiful minis, regardless.
Best of luck on your search.