Showrooms and auction houses are the best locations in Car Mechanic Simulator 2018 for getting cars in the best technical condition. Below you will find the descriptions of both types of locations.
In a showroom, contrary to junkyards and barns, you will find new cars. New both in terms of condition (100%) and in terms of models - you won't find any vintage cars, such as Bolty - rather something like Mayena. Not everyone will be able to afford such luxury, though. New cars can cost well above 100,000$.
Auction House provides the best balance. The cars there are diverse both in terms of price and age. The prices are pretty random - you have to be the last bidder to win an auction. The estimated price is based on the visual and technical condition of a car (which is also influenced by how complete this car is).