I have noticed that 7.92 shooting ammo seems to be drying up from all of the retail sites, anything worth paying for anyway. Gunbroker still has plenty come & go, but the key is using their notoriously picky search engine. On Gunbroker you can get potentially different results by using such search strings as:
8mm German, 8mm Mauser, 7.92mm, 7.92 mauser, 8x57, 8x57mm, 7.92x57, 7.92x57mm, etc… Some sellers will put spaces in the caliber when typing the title of the listing: 7.92 x 57mm , which will not come up in search results if you type without spaces. And then you get the unfortunate listers who will put an error in the title such as 8mm “mauzer”, or 7.29x57, or 7.92x75. You also get some very vague sellers who will put up a listing as “WWII ammo”, or “Nazi rifle ammo” or some such thing without mentioning the actual caliber, but which might contain a lot of 7.92. The Gunbroker search engine will not loosely interpret results in the way the Ebay search does, and so searching meticulously in many different ways may pay off.