I would guess ebay. Never heard of what part of  loco it was used in.

Rich

A few years ago SoundTrax came up with a line of motor only decoders. It was noted by many in other forums that they had issues matching up different decoder speeds. Check the SoundTraxx site The motor only match the sound decoders speeds.

Rich

Here is an active SoudnfTraxx Group I use to belong to for some years.

https://groups.io/g/SoundTraxxUsers

Rich

You could try a search at Walther's.

https://www.walthers.com/advancedsearch/search/index

Rich

You do not understand, SoundTraxx makes them to Bachmann standards. Cost cutting. Deal with it.
Just check the CV's. Many assumed.

Rich

To my knowledge they are what is called stripped down models. Not all features. That has been generally known for sometime.
I have an onboard 4-6-0 that I got about maybe eight years ago before this series and it did not have all the features of the Tsunami models I bought to install myself.
Some years ago when this started happening many thought they were being cheated since it was not a full Tsunami. I saw complaints in different forums. Some ripped them out.

Rich

Welcome.

Take some time to look through all the Bachmann site.

This link will show you all the CV's for different locos.

https://www.bachmanntrains.com/home-usa/dcc.php

I bought that loco about eight years ago with sound onboard but not sound value.

Good luck.

Rich

I am sure you will find some cars on ebay.

Rich

If it has the plastic clips, some remove them and solder the wires. The diagram if you do not have it.

https://www.bachmanntrains.com/home-usa/dwg/dwgs/HO_GE_ES44AC_(DCC_Sound).pdf

Eich

Do an Internet search.

Rich

 You need a photo hosting site.Here or any other forum.

Rich

A photo of your truck might help. It might be difficult to determine the age of your loco for some.

Rich

Maybe ebay but chances are slim.

Good luck.

Rich

Did you just put a dfecoder in? Usually when a decoder is pugged in wrong the loco runs in the wrong direction.

Rich

I think I see the power truck in the Parts list. The model has been discontinued but the truck is still listed.

https://estore.bachmanntrains.com/index.php?main_page=product_info&cPath=68_208&products_id=1519

No idea how old your loco is.

Rich