You should have no problems finding (with a *little* effort) W&ARR rolling stock to pull with your new General. I collect W&ARR stuff (and I have that General on order, as well), and here is a partial list of some of the modern production stuff in my collection. I didn't include the sets or other General engines. When it comes to the MTH, it is not complete (I think -- I may not have bought the most recent runs).
I also did not include any of the W&ARR Postwar Lionel, and there is quite a bit out there, as well. I also did not include the rolling stock made by SMR Trains as they are brass, very expensive, and not easy to find.
Lionel
9551 W&ARR Baggage Car
9552 W&ARR Passenger Car
9553 W&ARR Flatcar w/ horses
15029 W&ARR Wood-Sided Reefer
26765 W&ARR Wells Fargo Sheriff & Outlaw Car (Postwar Celebration series)
29459 Archive Collection 1878 W&ARR Water Barrel Car
31569 Western & Atlantic Passenger 2-Pack
35184 W&ARR Baggage Car
36657 W&ARR Caboose
36856 W&ARR Sheriff & Outlaw Car
39480 W&ARR Flatcar w/ Cannon Load
3949 W&ARR Cannonball Flatcar
52348 LCCA Halloween General Sheriff & Outlaw Car
52405 LCCA Halloween General Add-on Set (2 cars)
82447 Western & Atlantic Sheriff and Outlaw Car
83752 Western & Atlantic Horse Car and Corral
MTH
30-6400 W&ARR Overton Passenger 3 Car Set
30-6404 W&ARR Overton Passenger 4 Car Set (Texas Coach Cars)
30-6429 W&ARR Overton Passenger 3 Car Set
30-7114 W&ARR Stock Car (19th century)
30-7216 W&ARR Gondola Car (19th century)
30-7320 W&ARR Wooden Tank Car (19th century)
30-7439 W&ARR Boxcar (19th century)
30-7635 W&ARR Flatcar w/ Cannon (19th century)
30-7724 W&ARR Woodsided Caboose (19th century)
30-72084 W&ARR Gondola Car (19th century)
30-72125 W&ARR Gondola Car (19th century)
30-73410 W&ARR Wooden Tank Car (19th century)
30-74666 W&ARR 34' Boxcar (19th century)
30-76228 W&ARR Flatcar w/ Cannon (19th century)
30-76439 W&ARR Flatcar w/ Cannon (19th century)
30-77242 W&ARR Woodsided Caboose (19th century)
K-Line
K661-8011 W&ARR Flatcar w/ cannons (#30830)
AHM
7306C W&ARR Bobber Caboose
Here is a link to a Trainz article on Lionel Generals and other 1800's era sets that includes lots of photos of the various cars, including the Postwar cars.
Happy hunting!
Andy