As far as US Military surplus weapons go there simply are no more stockpiles of them. They where exhausted years ago. The exception to that general rule is the CMP which only sells what it returned to the Army by other countries and VFW groups.
Once our military adopted a new rifle they often just sold off the obsolete ones to the open market and did not generally stockpile mass quantities of them like the commies did. Now the former communist countries are clearing out their arsenals of the old stuck and we get to buy it all
Another is the general American philisophy on acquiring weapons. Contracts where extended when rifles needs and contracts terminated when no longer needed. A fine example is the M1 Carbine. Over 6 million where produced in a short 3 year period. At the end of WWII all contracts where cancelled and not another one was built. Communist countries but millions upon millions of weapons when they where not even needed, partly due to military doctrine, party due to socialistic work programs.
Also, in the grand scheme of things, we Americans just did not produce weapons on the scale of other countries. It is estimated that over 36 MILLION Mosins produced of all varieties. Germany alone produced close to 16 MILLION Mauser 98K's just during WWII, that doesn't even include the previous 40 years. Yugoslavia made millions of Mauser pattern rifles for years after WWII that were already obsolete and never even used, just stored for a NATO invasion. That's just three examples.
Off the top of my head America produced:
A little over 6 million M1 Garands
A little over 6 millions M1 Carbines
Around 1.5 million M1903 rifles
Just over 2 million M1917 rifles
There are plenty out there and they are all C&R, you just have to keep an eye out for a price you can live with. They are just no massive stockpiles left here or overseas to be imported.