for something like that, you can try looking for some ammo's forums, or maybe even a weatherby forum
places like Armslist com , will have free on line add you can place, and gets a pretty good amount of action
local gun shows too , you might find ammo booths
BUT your not going to get the best offers I think at a gun show, too many are there to buy and flip things IMO
gun show;'s are NOT as they were 20 yrs ago, when deals could be found
I don't know HOW rare your ammo is
but if its on the real rare side of things, some of the better high end auction sites can maybe get you top dollar
as a few do tend to auction off high end things and have a loyal following of buyers with deep pockets!