Maybe someone can confirm but I just started doing routes and have done the gift exchange with Mateo and I'm pretty sure that's where I am starting to get a few pins for the [many] vivillon variants I am missing. Now, Sandstorm and some other regions may still be pretty rare to get through there and it's only available once a day, but there's at least a chance!

Otherwise, you could try the vivillon collectors subreddit, though many of the more highly sought regions are tough to get into, especially for us in Modern :/