S-scale offers over 1,100 ready-to-run passenger cars, both new and second-hand. But, what if you want to model one or more passenger cars matching the prototype and those are not available commercially?
Bob Hogan has built many passenger cars for his own layout as well as for customers. While scratchbuilding is an option, he prefers, where possible, to start with commercially-produced replaceable car sides. These only apply to standard-gauge cars, not narrow-gauge, and are primarily focused on lightweight passenger cars.
Marker Light Productions, a company based in Wilmington, Massachusetts, used to produce brass, and later, aluminum sides for some Chesapeake & Ohio, Delaware & Hudson, and Denver & Rio Grande Western passenger cars. Due to increasing costs of the raw materials and problems with reliable production (the actual work was being done by a third-party company), Marker Light Productions stopped producing their kits. You may still find these on the secondary market. If you have any interest in these, get them while you can.
The Ohio-based company Laser Horizons is owned by Dennis Sautters. He produces laser-cut white styrene sides. His production method does not allow for contours, such as fluting, belt rails, and rivets. However, it does allow for the production of smooth-sided lightweight passenger car sides. The company has over 270 different sides available. Note that the company produces most sides in various scales, so when ordering, be sure to ask for the S-scale version.
Union Station Products, based in Meraux, Louisiana, also produces white styrene sides, but their production method uses a milling process. This allows for fluting. The company has over 1,000 different sides available in S-scale. On the company's web site, be sure to select "S-scale" in the drop-down selection field, as the pages default to HO-scale.
All of these methods require a core car body or donor car, i.e. the bottom chassis, the internal framing structure, the roof, the ends, wheels, trucks, and couplers. For passenger cars that are around 80 feet in length, Bob uses either the Chester Industrial Arts, J-C Models (both hard to find) or the American Models' American Car & Foundry passenger car bodies. If you already have any of these cars, you can use that as your starting point. If you don't, contact American Models to see if they will sell you just the core body (it is significantly cheaper than a fully decorated car). Note that, with the exception of the 1984 run, American Models' Budd cars are considered "shorty" cars, i.e. they aren't the typical 80-foot passenger cars that Budd produced, and so they cannot be used for these car sides. Also note that an American Models heavyweight passenger car cannot be used as a body core for a lightweight car.
Alternatively, Union Station Products produces a 3D-printed body core kit in S-scale, pictured below.

Bob Hogan worked closely with The Supply Car's owner Bob McCarthy to produce that company's Budd car kits. Basically, these were kits that consisted of the American Models body core, the Union Station Products sides, extra styrene for the various boards, diaphragms, roof vents, and grab irons. Underbody detailing, if the modeler wanted that, was up to the modeler to purchase from other manufacturers. When Bob McCarthy passed away, Bob Hogan took over the product line, and continues to make these kits available or you can commission him to build you these kits. Bob Hogan wrote the original instruction sheets that came with the kits, and he has graceously allowed us to post it on this web site. You can gather up the parts yourself and build the model, if so desired. He also provided a listing of the various underbody details for the different types of cars, including a parts-identifying image. Article and photos are copyright © Bob Hogan; used by permission.
Bob Hogan provided a collection of photos of cars he has built using the Union Station Products sides that represent the 1941 version of that train, as the train appeared in 1950 (diaphragms and center skirting had been removed). Article and photos are copyright © Bob Hogan; used by permission.
Bob Hogan provided a description of the cars that made up the SP's Shasta Daylight consists, a number of photos of cars he has built, as well as a scan of the page of his article that appeared in the November 2005 issue of S Gaugian. The train arrived in 1949 and Bob built his models as they appeared in the 1950-51 timeframe. He used American Models' core body and the Laser Horizons sides. Article and photos are copyright © Bob Hogan; used by permission.
Bob Hogan had an article published in the March 2006 issue of the S Gaugian magazine in which he describes how he built the sleeper cars that were part of the SP's Lark train. He provided us with the scans of that article. The photos of the article aren't very clear, so Bob also provided his original photos. Article and photos are copyright © Bob Hogan; used by permission.
Bob Hogan has done a presentation at an NASG Convention showing, via photos, how to build a The Supply Car kit. Photos are copyright © Bob Hogan; used by permission.
The Silver Planet was an observation car of the Western Pacific's California Zephyr consist. Bob shares his notes and a couple of photos of his build.