Ring! Ring! The 1950's are calling and they want their Warbonnets back! Okay, just kidding!
I think in the 50's and 60's they were probably one of the most produced paint schemes. My third-hand AF set that I got in 1968 was an ABA set of Warbonnets (a freight set.)
I really had fun with that set, but when I got back in the hobby almost 20 years ago, I found many other liveries that meant more to me.
And as noted in a couple of the post above, there have been a LOT of Warbonnet models produced by many of the manufacturers. If the current inventory of new trains are low, it's because there is not much current demand for them. However, I think this is why its the greatest time, ever, to be into O gauge trains, as there is some much (new and used) on the market.
Jim