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.
I assume you are talking about a 240v tandem breaker (2 240v circuits
in 2 slots)
It isn't legal but I used two of the tandem breakers with the full
size handle in line (not 2 little ones side by side) with handle tie
kits.
Folks install sub panels to solve problems like this if the breaker is
truly unobtainium.