The sales of dome cars relates in a way to the regular real estate industry. There are a few "players" like the real estate brokers with either cars for sale or rebuilders who also lease and sell, and occasionally a "FSBO" ("for sale by owner").
Additionally, there are what is known as "vest pocket" listings - i.e., not for sale, but "maybe if the right offer came along". In tracking the location of the cars, Phil "VistaDohmen", our historian, has talked with almost every owner of a car, and generally knows if an owner would consider an offer.
Cars that can be purchased run the gamut from virtually as good as the day they were built to rust buckets on their last legs, so you should have a "ballpark" figure on what you expect to spend for a purchase and what you expect to spend in restoration.
None of these car owners is interested in bothering with "lookie-lou's" or railfans, so unless you are a MAJOR corporation, expect to have to furnish some background and financial information before Phil will bother an unlisted owner.
If you are SERIOUSLY interested in purchasing a dome and cannot locate what you want through a dealer or on the open market, contact Phil at 400phil_AT_gmail.com to see if there's anything else out there that might be available.