Ammoseek is a great ammunition search engine that will look on the internet for all items available. I have use this to direct me to the best price option. Use the filter to get what you are looking for. You will pay $170 to $250 a flat for 3” 410 right now. 3 months ago there were more listed but Dove season seems to have cleaned up some of the offers.
Good luck.
https://ammoseek.com/ammo/410-bore?sl=3