You can check Scifinder or Reaxys. You would probably find list of other vendors

I've had great luck before using ChemSpider (http://www.chemspider.com/Search.aspx) - search by CAS number in the simple search, choose the appropriate compound from the results by clicking on the idetifier number to the left and there is a "Chemical Vendors" tab below that has useful links.  Good luck!

Thanks.

Search Zinc12 database with "purchasable" option

http://zinc.docking.org

There are so many as given before.

Please also add

http://en.wikipedia.org/wiki/Main_Page

www.drugbank

www.drugs

in addition to goole search, which contains all of compounds that might be missed in one of the search sites

Also worth exploring:

http://store.p212121.com/