.9mm, .22LR, .223 Rem, 7.62x39mm, and 12 Gauge would probably be the most common rounds.
.9mm - Surplus from Germany from use with the "Luger" as well as police side-arms.
.223 Remington - Civilian version of 5.56x45 NATO, US current issue and a very popular round, Military rounds aren't completely interchangeable with Civilian counterparts.
7.62x39mm - Surplus from several countries, used in the SKS starting in 1945 and the AK-47 starting in 1947.
.22 Long Rifle - Common hunting round since the early 1900's, you can literally buy thousands of them and walk out of the store with them in your coat pockets. Packaged in bulk.
12 Gauge - Common hunting round, can be very destructive. Cheap to make, cheap to buy.
...just in case anyone wonders why I chose those.