I was reading through some instructions on how to add a power off/on button to the Pi and accidentally misread instructions and shorted the two end pins on a 3+B model, which appear to be 3.3v and 5v. It doesn't appear to boot now and I just see a red light when plugged in. It was powered with a 1.8A USB power supply from an old phone. Is the whole board fried? Can I replace one component and salvage it? Should I keep it for parts or just recycle it all?
I did the exact same thing: Shortened the 5V and 3V3 rail by accident (with a probe).
In my case it was sufficient to replace the Power-Management-Chip, which is indeed available. I used aliexpress to order it (MXL7704-R3).
You will however need a hot air rework station. A cheap one as in my case will do, but a simple hot air blower from the hardware store is not sufficient (as it is too hot and too coarse). Of course, some soldering skill is required.
Please try this repair :) In the worst case you will loose 5 bucks but earn some experience. In the best case, your RPi3B+ is alive an well again for only 5 bucks.
I am afraid the only replaceable item is the Raspberry Pi.
It is almost certainly the new Power Management chip - which is custom designed for the Foundation. Even if you could get one, replacement is infeasible.
It is very well possible to repair this. The schematics for the board are available on the raspberry pi website and you can get the one for your model. Depending on your model, you may or may not have a fuse that links the fried components together. If the components did not have fuses, you will have to buy new ICs and solder them on weather thats a whole new CPU or a power management controller. The schematic is very detailed and will tell you resistance and how much voltage goes through each component. If you are not good at soldering tiny components, you are better off just buying a new one. This thing most likely wouldn't be salvageable unless you do lots of PCB work in which case a couple resistors could be of use but the rest should be recycled.