Best 1930-1940's era scale 40' flatcars currently available?

Well, depends on what you mean by “currently available “.  AHM and Pola made a pretty decent 40’ flat in the late seventies, but it was designed for 2 rail.  You need to add your own trucks/couplers and most likely repaint and decal into your preferred road.

Here’s one of my for sale cars up close with a Sherman loaded; my own wooden deck to cover the plastic deck of the model:

Here’s an overall pic of one of my for sale shelves with a AHM 40’ center front

These cars are pretty commonly available, and I would never pay more than $20 apiece for them.  I usually try to buy in bulk and get them down to between $10-$12 apiece, seeing as they need new trucks, paint and decals before they’re anywhere near ready for revenue service.

Hope this helps.  Any questions feel free to ask.

Cheers,

Price

While Lionel's PS-4 Pullman Standard flatcar is a postwar design, it is probably the nicest mass-produced flatcar in O scale.  Alternatively, the Menards 50' flatcars are surprisingly nice for the price.  I have several that I have plans to repaint for correct paint schemes. 

I agree with Johnathan on the Lionel’s as far as out of the box. I model a road with not much offered in 3RS. I look for Weaver’s with minimal lettering. I remove 2 unseen ribs on the chassis which allows you to really lower the car. Rough up the deck and paint it a wood color. Shave off some molded details. Add scale size stirrups, grab irons at the car ends and a new brake wheel with a more scale size shaft. Then re letter.

Addendum:

I would be remiss in not mentioning that Lionel acquired the molds for the AHM/Pola 40’ car and produced them as their Std O flatcar in the early 80s.  Lionel then kept using them (with some truck modifications) up to recently.  The most prominent examples I can cite are the Lionel Route 66 flats with ceramic (?) cars, and the “Peace Automobile Sales” flats with VW Beetles and flowers painted on the decks.  The bonus for buying the Lionel editions of these cars is they usually come with die cast trucks.

Cheers,

Price

@Dave_C posted:

I agree with Johnathan on the Lionel’s as far as out of the box. I model a road with not much offered in 3RS. I look for Weaver’s with minimal lettering. I remove 2 unseen ribs on the chassis which allows you to really lower the car. Rough up the deck and paint it a wood color. Shave off some molded details. Add scale size stirrups, grab irons at the car ends and a new brake wheel with a more scale size shaft. Then re letter.

I would love to see which ribs you are talking about and how you lower the car.