The article @matthewcoxy linked to is also part of what’s actually being checked when a 3D print model is submitted for review on 3DOcean.
You should inspect the item description to ensure the item was advertised as a 3D printable model, as well as the model itself, visually, through the preview images (or 3D preview, if available). You can then drop the author of the respective item any quick pre-purchase questions, if you have any doubts, via either their contact form (visible on the lower right sidebar when clicking their avatar, if you’re logged in) or through the comments section on the item preview page. This will ensure that you’re either good to go.
If you’ve taken the above steps, you can rest assured that even in the event a minor gap does come up, this is usually a really easy fix, involving sewing & welding a few polygons that may have been accidentally left disconnected, simply applying a shell modifier to the mesh, or at worst, a minor Boolean operator.
There are also quite a few plugins (particularly for SketchUp or Blender) that automatically fix non-manifold meshes, if it’s minor gaps and missing polygons or invisible faces. If you don’t have a suitable 3D package or the time to enable you to do this, again, the author of the respective item should be able to assist in no time
Last but not least, to your specific question: Yes, here’s one: https://3docean.net/item/aleysha-z-tool-/104695 Check the author’s profile page for more examples