Artworks made using alternative media are best accessed by but often exist only in the form of

Artworks made using alternative media are best accessed by direct experience but often exist only in the form of visual documentation or personal accounts.

Alternative media is popular with a certain niche of people - it is obviously not mainstream, so not a lot of people outside that particular niche know about it. It cannot be accessed in the 'normal' way, but rather only using various alternative methods.