Sometimes the sheer challenge of presenting a work and the exigencies of concert-planning can mean that even the most important ones exist more by reputation.
I haven't seen it yet but it's entirely in keeping with Varèse's ideas: another of his classics, Poème électronique, was written for the Philips Pavilion at Expo '58 in Brussels, where it was broadcast through 350 or 400 speakers in a stomach-shaped space designed by Le Corbusier's assistant (later a composer) Iannis Xenakis. Le Corbusier himself oversaw the accompanying images and film clips. Though the pavilion itself was dismantled, we do have various designs, notes and stills and a stereo version of Varèse's composition, so that it was possible to make a sort of virtual recreation.
Riccardo Chailly (carrying the imprimatur of Varése's amanuensis Chou Wen-chung) includes a couple of byways that are otherwise unobtainable and presents the original version of Amériques - Varèse's later, less resource-heavy version is generally performed. But even without those, Chailly's is the single set to get. For historical interest, you can also get the boxily recorded Frederic Waldman etc al "volume 1" (there was no volume 2) that inspired Frank Zappa.