I was wondering which file in templates represents cassus fett's pistol?

There are 5 Blasters that have the name Cassus Fett's Heavy Pistol; their ResRefs are g_w_hvyblstr05 .. g_w_hvyblstr09.

However, g_w_hvyblstr05 would seem to be the one that appears on the palette in the development system.

Each of the 5 have different properties and costs. Check them out in templates.bif with Kotor Tool.

You can find most items' ResRefs by navigating to BIFs | templates.bif | Blueprint Palette File | itempalstd.itp and double-clicking on the file. You can then use Control-f to find the entry you're looking for.

The other way is to open any of the Character Blueprints (.utc files) and click the Inventory button in the UTC Editor. Look for the name you want in the treeview, then drag it to the Pack Inventory. You'll see the ResRef for the item there.