Could the FBI find evidence without THAT FISC order? Sure- if they have the ability to get a FISC on anyone- they could have requested one for each of Trump's campaign inner circle.
No. Actually they couldn't do that. The topic was "collusion with the Russians" and Page was their best bet on that play. Except that his contribution to the Trump Campaign was very near zero. Never attended a committee meeting. Never talked to the upper reaches of the Campaign staff.
No way the FISC would approve a request based on Russian collusion for just ANY member of the team..
Page lived in Russia while he opened Merrill Lynch over there. Is a Russian expert on their energy reserves and had COOPERATED with the FBI many times in the past. They USED him as an "unwitting" informant in their zest to get dirt on the Republican campaign staff.