I’m trying to make a game in which one can purchase clothing off preset characters; would it be possible to use the ShirtTemplate/PantsTemplate IDs on the clothing to get the ID of the shirt or pants on the catalog? I’m aware of how to do the reverse (Shirt on catalog => ShirtTemplate)
There is no way to reliably do this. You can work around it by getting the template asset ID and adding one until you get to a shirt with the same owner. This is very inefficient and unreliable, but unfortunately there is no better way to do this. I recommend using a different system.
Note that there was a feature request posted in 2017 similar to this.