Hello. This has been a problem with those cars for a long time.
I think you will find the Lionel number you referenced above is "out of stock". I just looked on the Lionel site but I may be wrong.
Anyway, I have been looking for replacement trucks (side frames) for a number of years. I have not had any luck finding any. I have 10 cars currently in storage. If you do, please share with us.
Thanks.
Donald