I agree that the US with it's relatively lax weapons laws (compared with the majority of civilised countries) is going to have a higher proportion of ammunition available. However a 50+ year old Sherman with a full load of 75mm High Explosive (HE), White Phosphorous (WP), Armour Piercing Capped (APC), Armour Piercing (AP), Smoke (SMK), Chemical (CHEM), and/or High Explosive Anti Tank (HEAT) is just too unbelievable.
SOME rounds, sure, but only the basic types of solid penetrator (APC being a
possibility) - the others are sure to be rare in the extreme.
IF a community/organisation does happen to have a full ammo load, you can bet there won't be many/any explosive rounds in the mix, especially if the vehicle has seen combat.
Machinegun ammo on the other hand will be plentiful, provided the required belt link is available.