Unless I find out in an alternate way (2,3)
I believe the answer is:
if not
'unless' is the definition.
(thesaurus)
'i find out in an alternate way' is the wordplay.
'in an alternate way' means one should take alternating letters.
'find'+'out'='findout'
The alternate letters of 'findout' are 'fnot'.
'i'+'fnot'='IF NOT'
(Other definitions for if not that I've seen before include "Failing that" , "Unless" .)