There's an AliExpress link in the README of the Github repo.

Though, I'd imagine you could use ones from cheaper 'phone-to-face' Google Cardboard-style ones.

Lenses are actually not that irrelevant for this application. There is a huge difference between some cheap plastic and camera-grade glass. You don't want [excessive] chromatic aberrations, or [excessive] "God rays" (the Vive has that effect), or other optical artifacts.

Optics is similar to electronics: With high school physics it's all quite simple, but with high school physics a transistor can switch nearly infinitely fast ;-)

That's true. I actually have a few cheap card-like gadgets. And they're certainly not fresnel lenses. ;)