Kc-

I'm relatively certain you're on your own with this one, fortunately handloading slugs is pretty easy stuff and easy to set up for.

Cruise over to 16ga.com and ask the 2-1/2" slug/roundball recipe question and you'll get some responses, also solid leads on where to find whatever's available on the market.
I'm confident no one is selling loaded 2-1/2" 16b slugs though.
If it's out there, those guys will likely know.

--Tinker