Hey there--
You might check ebay for awhile and see what comes up under 'O' scale freight cars. You might find an old classic tank car in Mobil by Lobaugh, Scale Craft, Wathers, or All Nation/Thomas.
I recall seeing red ones occasionally in red with the Pegasus. If the ancient ones wouldn't fit in with your collection, if there are any in K-line---I think you can pull out the wheels and replace them with Athearn 2 rail wheel sets. For Weaver----the trucks can be easily changed to 2 rail wheel sets. The trucks are the same and wheel sets interchange.
You can find out right here in these forums---craftsmen who can easily change numbers for you. It would be somewhat silly, to pay the shipping both ways for such a minor alteration. Maybe you can find somebody nearby to do the job. It really isn't hard, Usually you remove one digit and replace it with a different one.
There are plenty of tips around here on how to do that; you can search them out. You might want to have a tank car custom painted and with custom lettering to your specs. Not cheap. But at this point the shipping cost is more justifiable.
Myself---I'd look for a lettering set of the same font, size and color at a hobby shop, gently remove a digit and rub on new one. If you goof, remove with scotch tape and try again. How badly can you goof applying a "1"?
This might be a good place for you to start tinkering with your equipment. Changing a digit is a baby step toward bigger and better adventures.
So I see many choices for you, from simple and cheap to elaborate and expensive. Soon other "rocket scientists" will chime in---and we will do our best to leave your thoroughly confused, (wink wink).
Have fun, fellow 2 railer------Rusty Rails----