A:Answer Hi I just wanted to share I found this on reddit.
The factory upgrade for the Inspiron 3880 is a 260 watt "bronze" model. I don't have the model number, but hopefully a helpful redditor will chime in with it. However, when searching the part number for the 200 watt PSU that came in my machine, I noticed ebay sellers selling it as an Optiplex 3060/5060/7060 PSU.
Light bulb moment: it's a shared part. Indeed, take the front bezel off and our Inspiron 3880 chassis looks almost identical to the tower version of those machines. So, I search instead for Optiplex 3060 PSUs. That leads me to the K38XP, a 360 watt model that also happens to have a 6-pin graphics card power plug. Hot diggity!
I haven't tried this yet but would be very interested to hear if any of you savvy computer folks can confirm this works.