Rail smith has some. Walthers did some. Decals are available from Microscale. Wheels of Time did some baggage cars that I am trying to find.
I think this is an interesting question. Kato made a bit of a unique decision by choosing the excursion scheme; 652 was in the maroon scheme during Rock Island days. I think some modelers will choose to use it as an excursion locomotive, while others may use it as a stand-in on their Rock Island layout. The Midland Railway owned 652 when it was in this scheme, and their excursion fleet is a mix of a bunch of cars. The photos on rrpicturearchives looked like a random mix of heavyweight and streamlined, but accurately modeling any one of those cars could be difficult. It may be easier to just go for the 'look' of it and choose similar cars and railroad schemes. I'm not a Rock Island modeler, so I'm not sure what are available as period-correct cars if you chose to use it on a 1950's layout. Any of the non-articulated Rocket consists would probably be appropriate to have behind an E-unit.
This is the Life-Like version with some Walthers cars. A smooth side baggage, a coach, and a 10-6 sleeper. These cars are stand ins for my version/representation of Rock Island Train #51/52, the Choctaw Rocket, along with a Bachmann observation car. The Bachmann Silver Series N 14554 85' Streamline Fluted-Side 3-2 Observation Car is modeled after a Rock Island prototype.
The Micro Scale decals for Rock Island streamlined passenger cars are: Rock Island Pre & Post-War Rocket Pool Passenger Cars (1947-1978) -- Stock# 60-1218 and Rock Island Streamlined Passenger Cars - Rocket (1937-1978) -- Stock# 60-1185 Both sets are listed as in stock at Micro Scale.
CC made push-pull commuter cars that could also be pulled with this. The trick is, you gotta' find 'em before I do !!! I have CC Set #1 (are there others ???) corrugated cars, several CC commuter cars, SHOULD have RailSmith cars by T-giving. Missed out on WOT baggage cars, so I'll be looking !!! The AB-6 would be VERY cool, but only 2 were ever made. They were used as a B unit for the Rocky Mountain Rocket 'til it got to Limon, CO. The train split and the AB-6 was used as A unit to Colorado Springs. They did get second prime mover before going to commuter service. For pike-sized trains, there were the Peoria Rocket and Quad Cities Rocket. E unit and couple of coaches. There was also a Rocket from Memphis, TN to Amarillo, TX. One RDC and a coach. Had to be a LOOONG ride !!! So nice to see interest in the older paint schemes. Never liked the blue / white scheme. It was a sign of bad times !!! Living in or around Joliet, IL forever, I've seen more Rockets then NASA !!! Always loved 'em.
I ordered a set of two coaches and a sleeper from Rail Smith. The cars look so much nicer than the Con-Cor units. Yes, they are expensive but cheaper than buying a CC Rocket set. I hope Railsmith makes some more of the cars that would work with this train. I have a feeling these are going to sell out quickly once these Kato locos get closer as there aren't really any Rock Island passenger cars available.