I've been buying mandarins from all over the place hoping to find a seed but no luck. I just ate a whole bag of satsuma's (origin: spain), didn't find a single seed. Are the days of seeded mandarins over?

Wait for clementines from California. Only very few of them have seeds, at most 3 seeds every now and then, but still considered seedless.

Many seedless mandarins came into production in California, specifically the Tango and Clemenules. If it is the Tango, it is truly seedless.

Bigger ones like Yosemite gold, Tahoe Gold and Gold Nugget, they're all seedless.