The most prominent place to see the symbol of Avacyn is in the iconography of her church. For example, there's this collar:
Avacyn's Collar | Art by James Paick
You'd think those collars would be unique to the priesthood, but they've also collared their griffins:
Silverclaw Griffin | Art by Daniel Ljunggren
Even Clerics who aren't working inside the church wield Avacyn's symbol. Thraben Doomsayer has a staff that should look very familiar at this point.
Thraben Doomsayer | Art by John Stanko
The obvious symbol on Executioner's Hood is on the wall in the background. But a closer inspection of the hood itself reveals that the symbol has been stiched onto the face!
Executioner's Hood | Art by Anthony Palumbo
The symbol of Avacyn is so widespread, it's even in common use as a gravestone, as seen in Grafdigger's Cage
Grafdigger's Cage | Art by Daniel Ljunggren
In fact, we've already shown you gravestones in that shape. Scroll back up to that Silverclaw Griffin and check out the graveyard. There are a couple of broken stones that clearly used to be in the shape of the symbol of Avacyn.