If you want to use the FIND function, the format would be something like:
IF(
OR(
FIND(LOWER(Source), "google"),
FIND(LOWER(Source), "bing"),
FIND(LOWER(Source), "search")
),
"Search Engine",
"Other"
)
I added LOWER so that capitalization won’t matter.
Or you could do this with REGEX:
IF(
REGEX_MATCH(LOWER(Source), "(.*)(google|bing|search)(.*)"),
"Search Engine",
"Other"
)
^ this is cleaner and perhaps a bit more reliable.