The CMP is the only major source of surplus M1s. The .gov owns the supply, and they release them via the CMP. Most foreign sources of M1s are countries that we lent them to, and when they finally decide they don't need them anymore, they give them back to the US gov, which then releases them via the CMP.
Most other types of rifle (mosin nagant, K31, etc) are sold by the governments that own them to private surplus arms sellers, which is why you see them distributed via many retail and wholesale outlets.
There are other resellers of M1s, but they're mostly re-selling guns that they bought from the private market.
Mike