I’ve done research into this subject myself, and I’ve come to the conclusion that he’d just have ESP send one with each guitar.
But he had an affinity towards the now discontinued ESP ES-S-20L. But any cotton ESP strap was probably right up his alley.
If you want to let the madness take you, you could hit up former COB techs Antti Härmä, Ari Arvola or Bobby Niemenen.
Fun fact: I’ve had a standing order with Thomann.de for an ES-S-17L and ES-S-20L for almost a year.
They also fucked me over when I ordered the EMG ALX set. See if you can find the straps on eBay or something.