Following up on this for those who are interested.

I contacted PreSonus support to find about parts and was sent to Fender, who ultimately sent me to a service center. I was not interested in going that route (effort, calendar-time, expense).

So I opened it up, found the part, and replaced it myself. To open it up:

1. Take off the rubber feet.
2. Remove the 4 screws from under the feet.
3. Remove the forward 2 screws in the center (with the rear of the unit facing away from you).
4. Do not remove the other 2 screws. If you do, it isn't a big deal, but it slightly complicates re-assembly.
5. Remove the 3-screws on each side.
6. Do not remove any of the screws on the rear panel.

The part I needed was for the little blue switches, not the back-lit source buttons. Those parts are titled "Antrader 30 Pcs Micro Push Button Switch, 6 Pin DPDT Self-Locking Power On/Off Latching Switches 8 x 8mm" on Amazon.

Now it works perfectly and was well worth the minimal amount of effort!