Learn English – Ask for help, or ask help

The first sentence, using "in search of", is the correct one.

1. "in search of" is an idiomatic phrase, equivalent to "searching for". These mean the same thing:

   I am in search of a good pair of shoes.

   I am searching for a good pair of shoes.

2. "in search for" is not strictly correct English; there should be an article ("a" or "the") before "search". Leaving articles out is common when trying to be brief, like in news headlines, as seen in Maulik V's answer:

   FBI involved in search for brothers missing in Matamoros.

   The FBI is involved in the [or a] search for two brothers who are missing in Matamoros.

These other two cases are not directly relevant to your specific question, but may help to clarify general uses of "search of/for", when not preceded by "in".

3. "search for X" means that X is the thing you're trying to find.
4. "search of X" means that X is the location where you're searching.

   My sunglasses are missing. A search of my car did not find them. The search for my sunglasses is still going.

It might be easier to start by asking where they work within the company; that is, ask a question that goes something along these lines:

Which branch do you work for?
What section do you work in?
Which department head do you work for?

One or two of those quesitons might give you the answer that you seek; the person might say:

I work in finance as a programmer in the payroll department.

If not, though – if the person just says something like, "I work in finance" – you can always ask a
follow-on question:

And what's your role there?
And what do you do in that department?