Here's some of my favorite 3d printable models that only need scaling to work in 15mm:
Dragon's Trapper Lodge Gorilla - use as Kong.
Requires no scaling - the default scale gorilla is around the size of Kong from 2005 Peter Jackson's version for 15mm figures. The same sculptor has a
more modern take on Kong (from the new Monsterverse movies) if that's your style, but that one would need to be scaled accordingly. In fact the whole
Lost Continent and
Simiax Legions lines from this sculptor are worth a look. Bunch of models would fit right into a pulpy lost world campaign!
Zombies and other Pulp/Occult/Sci-Fi Nazis from Kyoushuneko Miniatures.
This sculptor makes weird WW2 miniatures for different countries, but their German line has a decent selection that would fit right into Pulp (zombies, werewolves and scientists). The good thing is that the sculpts are pretty chunky and don't include any overtly tiny/fine detail, so simply scaling them down would work decently well for 15mm (although I would thicken them to be safe, little worried about the fingers snapping off. If it happens with zombies, it won't be much of a problem. Apart from the fingers, the figures are hefty enough to not break easily).

Vehicles from WindhamGraves from his
Thingiverse and
Patreon.
Windham focuses on XX century vehicles in 1/100 scale (I usually scale his stuff 110% to match the chunky 15mm minis better). While a lot of his stuff in not suitable for pulp (bunch of it is military WW2 stuff), there's decent amount of it that fits:
Ford Model T,
Model TT Truck or the
Liberty Truck will work in many pulp scenarios. He has more suitable models, those three were just on top of my head - check out his whole catalog on
this spreadsheet.
Crooked Dice U-boatWhile Crooked Dice offers a good selection of pulpy miniatures, just like most miniature stl files out there they are scaled for 28mm and would require a bit of "chunkyfing" to work well on 15mm scale. However, their terrain and vehicles would scale well enough. The U-boat is a staple of pulp, and their model would scale down to 15mm without much issue. You'd probably want to print the railings on a resin printer to be safe, but the u-boat itself would print just fine on an fdm printer.
----------------------------------------------------
That's all the models that would work well just by scaling the 3d files down to 15mm I can think of right now. All the other models I have need a little work to work at this scale.
----------------------------------------------------
Bonus you need to really scale UP this one! 
Skyfox3Ddesigns offers a free
1/350 Skycarrier Graf Zeppelin II! It's a huge model as it is, and it would need to be scaled up around 350% to work with 15mm figures, but if someone is crazy enough to attempt such huge project, it would make an awesome setpiece for a pulpy battle in the skies!