Ok so I'll try to keep it succinct, if this doesn't make sense, then it might be best to just attach the save or zip and we can find it for you. The very short summary is the save file is in XML format, which means you have nested attributes. So for example, there's a spot for a location, in this case the island farmhouse, and it has a bunch of attributes associated with it, one of which is the items in that location:
View attachment 25042
And you'll notice one of those item types is a chest. Note that it has a key / value pair, as well as a "tilelocation", and they match. Both show *where* the object in question is, so this chest is at the coordinates of X = 17, Y = 16. I don't know which direction it starts from with certainty but I presume the bottom left corner is x and y of 0 based on general math conventions. Either way it should be easy enough to find, only 4 possible locations based on that, if anyone knows for certainty let me know. Inside the chest, it has its own list of items:
View attachment 25043
So apparently in this chest in the IslandFarmhouse map is a stack of 3 iridium quality (quality 4) Flounders.
The point of this being, you can work backwards from any item. Search for the item name. Thankfully CA is WAY better than even major developers (*cough* looking at you Microsoft *cough*) at ensuring the "Graphical User Interface" or "GUI" name lines up with the internally used name for objects. So just search for it in your save file. Then, search "up" for "items" to find where the beginning of the item list is for the container it's in. Search "up" for the "tilelocation" to get the location of the container, and just above you'll see what it is, whether that's a chest or whatever else. Search "up" one last time for "gamelocation" to find the game location in which the container is located. Voila! You know what you're looking for and where, and should be able to find it easy enough.
Note that it'll look WAY less pretty than this by default, this is with an XML formatter being used. Specifically in my case VS Code is the program and XML tools is installed as an extension, formatting with shift+alt+f. But the principles are the same whether you're looking at it all nice and pretty or all messy in something as basic as notepad. I hope that helps.