I don't think JSWrights do that, because then folks would only ask/pay for premium light weight clefts, and there'd be no takers for the rest. I believe they only sell big lots with several grades mixed in (not even G1 only), and the lightweight clefts might be in there somewhere, not necessarily G1.
Lightweight clefts are a freak of nature, I don't think you can specifically grow them. They just happen.
You might have better luck scoring one of those with batmakers who have their own willow supply, such as Black Cat, H4L etc.