Below are some models available in HO for Santa Fe combines.

Rivarossi produced this model and they are very plentiful. Lambert/Suydam also produced this car in brass. Cars 2602-2608 were produced by the Santa Fe specifically for the Fast Mail as rider cars. They would not have been seen in branch line combine use. There are some details that are inaccurate. The 2602 resides at the Texas State Railroad in Rusk, TX.
For the scratchbuilder, the Rivarossi car is a good strarting place for a 72' 2548 combine. The 2548 in coach green paint lasted until 1971. Photos of both sides are in Coach, Cabbage and Caboose, p. 144, 152. Drawings of the original 1332-1341 Buffet Library cars are in the Society Diner book, p. 44+. The Rivarossi 2602 is a 77'6" car, requiring removal of over 1/2" of length between the passenger section and the baggage door. The 2548 is one of the few combines with the broad 6 window baggage door. Another combine that can be fashioned relatively easily from the Rivarossi car is the 2625 (p. 75, 132, dust cover) in Mineral Brown.

Pecos River Brass has produced several combines that can be found on Ebay and in some shops. The first is Santa Fe 2410 which should be painted mineral brown as 2410-2415. These 6 cars, 70' long, were converted from 3100 series smoker chair cars in 1956-57 and were the last combines converted by the ATSF. Photos in CC&C show 2410 in Pratt, KS, 1958; 2411 in Hutchinson, KS in 1960 and Anthony, KS in 1963; 2414 in Jetmore, Boise City, and Satanta, KS, in 1961. The 2412 was converted to MOW 195654 in 1964.

Note that this model includes an antennae on the roof

A second Pecos River car is 70' combine 2544-2545 which should be painted Coach Green. The 2544 is preserved today at the Illinois Railroad Museum in Union, IL. These cars were sectioned for smokers and non-smokers plus the baggage section. These cars were built in 1927 and retired in 1969. A photo in CC&C shows 2545 in Amarillo-LaJunta service in 1965.

Lambert/Suydam also produced a model of combine 2544-45 but their model had several inaccuracies which need to be corrected. For more details see the Lambert/Suydam reviews. These are readily available at reasonable prices and offer an inexpensive method of having a stand-in combine. I know of no actual prototype for this model.

Walthers produced kit 6653 of this 75' Double Vestibule combine in the 60s. They marked it as ATSF 2622-2643. The numbers need further clarification. 2622-2643 were converted at Cleburne between 1950 and 1953 from all steel chair cars, coaches, and smokers. Walthers assumed that because a car had 5 windows on one side it had 5 on the other - wrong. 2626, 2627, 2631, and 2640 probably had high mid-car washroom windows on one side. 2623, 2624, 2628, 2636 and 2641 had 4 windows on one side. 2633 and 2635 did not have the upper windows plated. 2638 had five windows on one side that match but I have not seen photos of the other side. 2625 appears to be a good candidate for this window arrangement. 2625 is shown in CC&C in Hutchinson-Wellington service in 1962. I have not seen photos of 2622, 29, 30, 32, 34, 37, 39, 42, or 43 to determine if they are candidates or not.

Key Imports produced a model of 2626-27, 31, 40 in brass. 2626 was the same series modeled by Walther's above but with a different window configuration. The 2626 served in Dodge City, KS, in 1961.A photo of 2627 at Amarillo in 1961 is in CC&C p. 134. 2640 is shown in ATSF Color Guide to Freight and Passenger Equipment page 22. It was awaiting disposition in Chicago in 1969.

Another interesting car that comes up on Ebay and in shops from time to time is the Hallmark model of the combines converted from drover cars. 2312 resides at the Great Plains Transportation Museum in Wichita, KS.
Check on this site for more information on these cars.