Here is an economical replacement for missing brake wheels: 8mm metal dress snaps.
These are found in the sewing department of many stores such as Walmart or Amazon. I found a card of 24 for $6.99 on Amazon. They come in 4mm, 6mm, 8mm, and 10mm. I use 8mm for HO scale.
The measurement specification is for the (unused) larger, female half. Buying the 8mm size will provide a 8mm female (unused) and a 6mm male (which is about the right size).
Just separate the two halves of the snap. Discard the female half. Enlarge the hole in the freight car if necessary. Put a drop of Krazy Glue in the hole, then press the male half of the snap into the hole.