“According to the 1997 Survey of State Prison Inmates, among those possessing a gun, the source of the gun was from –

a flea market or gun show for fewer than 2%
a retail store or pawnshop for about 12%
family, friends, a street buy, or an illegal source for 80%

During the offense that brought them to prison, 15% of State inmates and 13% of Federal inmates carried a handgun, and about 2%, a military-style semiautomatic gun. “

http://www.ojp.usdoj.gov/bjs/guns.htm