On Sat, 19 Oct 2019 22:13:31 -0500, Dave Garrett
wrote:

Anyone know of a source for old/obsolete breakers? One of the breakers
in the panel at my parents' house failed. It's a circa-1964 Square D
panel, model QO-8R0, and the breaker is a 15A 120/240 twin/tandem one.

The failure mode is that the breaker in question tripped and cannot be
reset. An electrician confirmed that it needed to be replaced, but
hasn't had much luck finding a suitable replacement locally. There are
no known issues with any of the other breakers.

Bay Breakers/Bay Power seems to be one of the go-to sources for obsolete
breakers, but they haven't responded to a message I sent via their web
contact form.

Anticipating the inevitable recommendation, upgrading to a new panel
isn't in the budget right now.
The QO breaker today fits right back to 1955 QO panels. QOB breakers
may not fit the older panels - not sure - but those are the tandem 120
volt ones - not 120/240