I get mine from multiple sources...
I use K. van Bourgondien quite a bit. I get Cinnamon, Tennessee Ostrich, Christmas, Lady, Maidenhair and Sensitive from them. I always wait for a sale. But, even at $15.00 for 5 Cinnamon, it's still cheaper that some nurseries.
I'll usually get a bunch of the pre-packed dormant ones in early spring. I also watch the clearance racks. Usually, I can count on getting some for $1-$3 on clearance.
Pre-packed dormant in early spring
I generally don't get much fern from nurseries due to the cost.
NGA-MAG Plant Swaps:
We here in the the Mid-Atlantic Gardeners group have twice-yearly in-person plant swaps. I usually get a few fern each time.
Swaps with friends:
Occasionally, I know someone that really has an overabundance and just want to get rid of some. (never turn down free fern!) I'm hoping to pick up some bracken fern next month.