If it’s not too much trouble, can you please tell me where in the Gemara or Rishonim that Argaman is also sourced from a Chilazon? Or is it my imagination that it’s mentioned somewhere and we basically need to derive that from the word Purpura?

It is not mentioned in a single place in the Gemara or the Rishonim to my knowledge.

It all comes from being based  ספרים חיצונים

Such as וכדומה  יוסיפים

But in my opinion, there is ample evidence to 2 things:

1. There is no Halacha that dictates as to what the source of Argaman should be, rather, if it is the proper color it is fine whatever the source is.

2. High quality Argaman was generally sourced from the Purpura species. But that’s all theory. See השתלשלות חומט המורקס טרונקלס בדברי גדולי הדורות

I have not seen or heard of any proof for the word purpura meaning any specific color to my knowledge.
I have seen it being used for 4 colors blue purple red orange, and saw that it includes anything in between purple & pink.

So when people say it was used for purple because it says purpura they are not really saying anything with substance.