First, if you are a U.S. citizen with a clean record, go to
www.odcmp.com for the best M1 bargain around. But read the requirements.
Other than that, M1's are available at gun shows and sometimes at gun shops. Original or rebuilt GI guns in good condition will go for $750-900 depending on condition. I would avoid those with new cast receivers marked "CAI" on the left side. Some new guns (or at least receivers) from Australia are now available; they are reported as not up to the standard of the wartime rifles.
Original M1 rifles have the marking on the top rear - only Springfield Armory, Winchester, H&R and International Harvestor made M1's for the U.S., Winchester only in WWII, the others in both WWII and the Korean War period. The commercial Springfield Armory made a few; these are not bad, but again not as good as the originals.
Also avoid any rifle that shows signs of welding; look along the bolt raceways and along the left side for any humps, roughness, or pitting that would indicate welding. These receivers were pieced together from the parts of receivers that were cut in half an scrapped. They may not be unsafe, but they are not as good as one that has not be so treated.
Do not believe any stories. Almost NO M1 rifles were brought back by WWII GI's. It was illegal and could (and did) earn a prison term if caught. Patton did not carry an M1; neither did Robert E. Lee or George Washington. (I bought U.S. Grant's M1 last year and it is not for sale. ;>) )
Jim