Grant
I dont know of any plastic B-7 kits. I am sure if there was one, I would have one. I try to keep close tabs as to what is and what has been on the market.
But dont worry, you can build one. I would suggest that you begin with a LaBell Alamosa kit and modify it to the B-7. I am not sure about the length, but it must be close. The LaBell Kits are fun to put together, but they do take time.
You could also try to modify a Model Railroad General Store passenger car. It would require changing the windows and some other changes. I dont have any of them on my railroad because I do not like the way the roofs look.
Your third option is to scratch build one. I have build a couple of passenger cars based on the LaBell cars I have. I basicly copied them with a few changes. Not to hard, but the roof can get difficult.
Best of luck
John Vandenberg