Ivermectin for human use is pretty universally available at pharmacies in the US, however requires a prescription from your physician. S/He would need to have read the manuscript presented in the first post, and agree to off-label use of the Ivermectin. You would probably need to sign an informed consent document that presented the various risks (IMHO these are minimal) versus benefits (Possible prevention of Covid-19 disease if exposed) for the use of Ivermectin. As previously mentioned, off-label use is both ethical And permitted provided there is evidence for potential benefit and there is no other definitively proven and widely accepted treatment protocol. IMHO waiting until a patient is at deaths door at a hospital is a bit too late to begin treatments - particularly when a known complication of Covid-19 (Cytokine Storm) can produce serious and long-term damage to major organs.

Be aware that using a crop lens on a full frame body, the lens cannot fully illuminate the camera sensor, so you get vignetting, loss of the corners as you see in the photo. Going to crop mode on the camera simply crops the resulting image so you don't see those corners, but the result is an image of much fewer pixels and the resulting resolution then the full capability of the camera.

Of course, that resolution was considered top of the line several years ago. The 18-400mm (or even 18-300mm) is a nice "all purpose" zoom range for the camera and is not available in a full frame version.