You generally won’t find 3D-printed guns for sale in a traditional store or online marketplace like eBay or Amazon.
3D printing a pressure-bearing device is inherently dangerous. buy 3d printed gun
Sites like DEFCAD or Odysee host the blueprints. Note that the State Department has previously fought to keep these files offline under export control laws (ITAR). You generally won’t find 3D-printed guns for sale