Doubt you will find just the brush holders so unless someone happens to have a motor laying around that is damaged or otherwise not easily repaired for some other reason you will need to get a complete motor if you want to stay Packard original. Gerry at PackardsSouthwest might have one but if not, our other vendors or those with parts cars or cars converted to 12v should be able to come up with one.
If you are not a purist and do not mind modern many vendors sell a universal 1/4" diameter shaft heater motor that in most cases bolts right in. It is a permanent magnet motor so is reversible by changing polarity. You need to make sure of the proper rotation when you wire it in the car and unlike a stock car, it will be an item affected if the battery polarity is accidentally reversed.
Here is one vendor that carries them but there are many others. While not needed on a heater blower wheel, some like this motor come with the adapter needed for the 5/16 bore wheels used in many AC units.
https://www.classicparts.com/1947-54-Heater-Motor-6-Volt/productinfo/68-005/#prettyPhoto[gallery]%20nofollow/0/This Post was from: https://packardinfo.com/xoops/html/modules/newbb/viewtopic.php?post_id=260842