The Henney Company in Freeport IL built pro cars almost exclusively on the Packard professional car chassis until 1954, and Packard itself folded shortly after that. The Henney Company briefly considered building on the Cadillac commercial chassis when Packard folded, but they ultimately decided against it, as they didn't want to build "just another Cadillac".
The Flxible Company in Loudonville OH built professional cars mainly on the Buick chassis until the end of the 1964 model year when they exited the professional car building business for the second -- and last -- time. They continued building Flxible buses for many years.
The Eureka Company in Rock Falls IL built only on the Cadillac professional car chassis in their antiquated two-story plant until they went out of business in 1964. Eureka's frames were largely made of wood for many of the years they were in production, and there remained some hand-carved wooden frame components even in its final years. Eureka was known for their high quality (and premium priced offerings), much of their vehicles were built by hand, and their interior appointments and components were made of the highest quality materials, with rich hand finished wooden components and cabinetry. Eureka interiors were, and are to this day considered second-to-none.
The Hess and Eisenhardt Company of Cincinnati OH, builders of S&S professional cars, built exclusively on the Cadillac professional car chassis. S&S professional cars were among the highest priced offerings, but many of their customers were all too willing to pay that premium price for the status and prestige of owning an S&S. They were known to utilize very conservative styling, and their hightop professional ambulances utilized essentially a largely unchanged body from the mid 1950s until S&S ended their professional car portion of the business in 1976. But their parent company -- Hess & Eisenhardt -- is still in business to this day, and is known for building highly specialized armored cars and limousines. In fact, the Hess & Eisenhardt company was responsible for designing and building president John F Kennedy's infamous convertible parade limousine, originated as a 1961 Lincoln, in which he was assassinated on the streets of Dallas TX on November 22, 1963. JFK refused to have the "bubble top" on the car as it made its way to the Trade Mart in Dallas, in a top-down motorcade. While the bubble top was in no way armored, or bullet proof, it may have provided some level of protection as the motorcade wound its way through Dealey Plaza in Dallas. This was the last time a U.S. President was ever allowed to ride in a top-down un-armored car. The famous Lincoln parade limo was eventually brought back to Hess & Eisenhardt headquarters, stripped down to the frame, and re-built from the ground up and completely armored, and a 1963 front clip replaced the original 1961 front. This car was continued to be used for many years to come, and all presidents were known to have used it up through Gerald Ford. The car currently resides in the Henry Ford museum, in Dearborn MI where it is prominently displayed for all museum visitors to see it.
The Superior Coach company of Lima, OH built both Cadillac, and Pontiac Professional Cars. The Cadillacs utilized the Cadillac professional car chassis and were built at Superior's Lima OH facility, and their Pontiacs were built on the Pontiac Bonneville sedan chassis at their Kosciusko MS plant. Superior ended production of their Pontiac pro cars in 1975, and the entire company went out of business in 1980. The U.S. Navy military ambulance that carried president Kennedy's dead body and casket from Andrews Air Force Base to the Bethesda Naval Hospital for autopsy and embalming was a 1963 Superior Pontiac ambulance, built using military specs, and was eventually crushed under orders of Robert Kennedy, who didn't want it to become a morbid curiosity.
The Miller-Meteor division of Wayne Corporation -- previously the Divco-Wayne company when they first acquired Miller, and then Meteor -- a bus manufacturer, built exclusively on the Cadillac professional car chassis until going out of business in 1979, not long after GM dramatically downsized their cars for the 1977 model year. The Miller-Meteor professional cars were ultimately built in the Piqua OH plant after a short stint at the A.J. Miller company's original plant in Bellefontaine OH. One of the more famous Miller-Meteor cars was a 1959 M-M combination car that was used in the 1980s hit movie "Ghostbusters" starring Dan Akroyd and Bill Murray. The car -- known as Ecto 1 -- has been recently restored back to production quality, and admired by Dan Akroyd who came to view the restoration, and reminisce. The Cotner-Bevington division of Wayne Works in Blytheville AR -- originally the Comet Coach company until Ford bought the Comet name -- built mainly on the Oldsmobile Ninety-Eight sedan chassis until they closed in 1975. An early 70s C/B (Oldsmobile) hightop ambulance was featured in the 1976 movie "Mother Jugs & Speed", and was operated by the fictious Unity Ambulance Service in which their rival company, "F+B", competed against them and jumped each other's ambulance calls often. Cotner Bevington also built a limited number of Chevy professional cars on the Chevy passenger car chassis, of which PCS President Paul Steinberg owns a 1962 Cotner Bevington Chevy ambulance built on a passenger car chassis.
There were a Hodge-Podge of smaller, less known builders as well, like Pinner Coach, National, Siebert etc, many of which would build on any chassis the customer requested.