It sounds like you're facing the common challenge of finding specific On30 passenger rolling stock, particularly in matching schemes. Here are some suggestions to help you locate the combine, coach, and observation coach you're looking for:

Check Specialty Hobby Retailers
Many hobby shops carry or specialize in Bachmann On30 rolling stock, often stocking items that are out of production elsewhere. Try:
TrainWorld: Known for a wide range of model train supplies, including Bachmann On30.
MB Klein (ModelTrainStuff): Frequently carries passenger cars, including older and hard-to-find stock.
Nicholas Smith Trains: They have an extensive collection of rolling stock and accessories.