Where do I find information on the competitors in a certain industry?

that's hard job to get accurate information but still one can use the following channels;

• published data of industrial associations
• Chamber of Commerce for Public Limited companies
• Vendors if supplying goods to competitors also
• Reports from Market Research companies
• Custom department data for Imported Materials
• Excise duties paid by the competition
• Periodic reports of your sales force 

The Best place to know the activities of your competitors is the Market Place. and no other place you can get so much details. 

Simply Google its name and start surfing. There you will always find you request.

1. Capital Market Information.

2. Business Periodicals and News papers-especially advertisements

3.Client Information

4.Supplier Information

5.Websites/Internet

6.Informal communications.

7.Telephone Directories and Yellow Pages.

8.Special Business Directories.

9. Brochures  of other suppliers with specific brand names

10. market intelligence agencies/ business bureaus/chamber of commerce and business associations for the particular industry/Business Exhibition / Permanent business exhibition centres.

starting with their web site is the best place , also a visit from any of your friends could give you a hint ,

following their projects or products in the market also helps .

Browse public documents, find competitors job advertisement, participates in the trade show, read the local & national newspaper. This might help you to find information on the competitors.

company profile history,public releases,financial statements,

Consult with any of the research companies, for public limited companies you can analyse their statements.

What kind of info? Please specify. To collect The basic info you could use the means, Mr. Vrindavan has mentioned. Depend on size of your comany or the kind of product you are intrested of. If you represent a coorporate, you could establish a inteligience unit, or hire a research expert with proven records. if you are in retail sector you can collect info for your competition based on sales team network, which has direct feedback from costumers. You also can invite your consumers to a social event or product presentation event to mine their opinion on the market and competition.