That is the best place, unfortunatly.
If you want to reduce corruption, you can aid that process with stuff like the fountain goddess, to ease the use of winterstem.
If you've unlocked the fox-house you can check in with the merchant there, winterstem is part of their rotating stock. If you have any Broken Metal Parts available all the better.
Quickest way? Old Forest; the Alraune worshippers can drop it and it's fastest and free.
Another one is I often get Winterstem from Vaush and his naïve cohorts in the Foothills, even if it's a fight between Brazenberry Ale and Conjurer's Concoction/Conjunction.
Fastest way would be what the others said: Kohaku in the Fox's Den in Frostwood - she's an adorable shorty too.