I have bought an old building and I wish to renovate it so that it can be used for commercial offices. I am in the process of locating and hiring a contractor for the overall renovation work. Any suggestions where can I find a reputable contractors? Thanks in advance.

You might check with the local building codes department. They should have a list of licensed contractors in your area. If not, check with the local builders' associations, chamber of commerce, banks, etc.
Then, once you have a few possibilites, check several of their references.

How big is the building and where is it? I can’t really recommend anyone if I don’t know what type of project your going to undertake or where, but I would recommend going to http://www.jobtrio.com the site is a median that helps consumers find contractors, there are thousands of contractors listed on the site so you should be able to find the right contractor for your specific project.

Word of mouth, check with friends, family and neighbors.

Your approach is backwards!

You're buying a building, and you want to make it so it "can be used for commercial offices"???

Did you even check your local zoning to see if this is possible? Did you just buy and decide to renovate?

You need to visit your local building and zoning department and ask some questions....BEFORE you hire a contractor! Now, there are contractors out there who know how to investigate these situations, but in the case of multi-story buildings, where you now enter the realm of "mixed use-separated....or mixed use-non-separated" you can't just say do this and I'll rent it out!

You may need a designer or architect depending on your local requirements. Either of these pros will check first what you have, then help you design for what you're allowed.

Good Luck!

Look here, there is a contractor "directory" look at the top of this page.