Although the large retailers don't have .303, with the exception of fair ammo from Samco, there is still lots of it in private hands that comes up for sale frequently in small and large quantites. There is lots of Vickers belted MkVII still out there as well as 32rnd boxed MkVII in military cases. There's belted MkVIII as well. Prices seem to be in the $.35 to $.50/rnd range for the belted ammo.
A large quantity of 8X56r came nto the Us about eight/ten years ago, but since it has waffens on the headstamp so may sellers raised the price to unrealistically stupid levels. That mmo is still out there as well and isn't being used up by the MG shooters since there are virtually no MGs in the US in that caliber.
As far as 7.92, there's still huge quantities in eastern europe, whcih will come to the US, and still quantities in South America, althugh the drug treaties with some of those countries prohibits the export of ammo to the US. However, surplus 7.92 will be around for quite a while yet.
Black River Militaria CII