Does anyone know of a 2 1/2" buckshot
OP are you measuring the loaded shell? because that would measure 2-1/2". Shell length is the FIRED length, so your normal loads that you are calling 2-1/2 are more likely to be the normal 2-3/4" ammo everyone is talking about.
If you can't find buck, get some goose loads - at HD distance they WILL penetrate