It does all seem to boil down to what a bartender can get their hands on
doesn't it? In London there are several companies who will sell you 1kg blocks
of frozen passionfruit puree, at a reasonable enough price.
If a person uses different spirits in a Sour, they are changing the flavour
profile of the spirit element, so why not with the Sour element?
Cheers!
George S.
"You have your way. I have my way. As for the right way, the correct way, and
the only way, it does not exist." - F. Nietzsche
"innovation distinguishes between a leader and a follower" - Steve Jobs