Sources of build-your-own rolling stock kits

You can still find Intermountain kits on the auction sites. They are plastic but the detail is probably some of best out there. Many pieces are delicate though so you have to handle with care. Their O scale kits came with both two and three rail wheelsets but ask before buying as many times one set of trucks may be missing. Same for Red Caboose but you will have to find other three rail trucks for these as they only came with two rail trucks.

Pete

Most kits in the older era came without trucks and couplers.    They met NMRA recommendations for height etc, and so so you simply put the trucks and couplers of your choice on them.    It did not matter what kind of track you used or 2 or 3 rail.

Labelle Woodworking makes craftsman wood kits that build up into beautiful early 1900s cars.    I have built a few and owned a few more.    They are a challenge, but if you enjoy it, make very nice cars.

Ye Old Huff & Puff makes craftsman wood kits also.    They purchased the old Mainline Kit Line and have continued as of the last time I looked at their website.    They also had kits of their own, maybe based on the old Boxcar Ken line.    Their line is much broader than Labelle.    Most of their stuff was also Silk Screened sides, no need for decals, but you have to match paint for ends and roofs.     These are nice kits too.    I have built a few.

I B.T.S also offers kits.

All of these have websites.

If you want to shop a little, attend an O Scale 2 rail show.     There are always stacks of older kits for sale and the price is usually quite reasonable.    There is the March Meet in Chicago, the Cleveland show in Nov, the Indy show in September and a quite a few on the East coast.   I went to The the Strasburg Meet in August and it was a very good meet for 2 rail O scale.

I know there are some West coast shows also, but I am not familar with them.    Also I think there has been one in Texas and in the Southeast.

I forgot to mention FUnaro and Camarlengo, whose kits are cast resin.   Below are some links.

http://www.fandckits.com/Index.html

https://www.labellemodels.com/

http://www.btsrr.com/btscar01.htm

http://www.yeoldehuffnpuff.com/

I think there is also a company called St Charles Model Works which does cast resin kits.