First, go to the brand or company web site (not the corporate one).
Check to see how many brand extension there are. Mars Inc, a private company, has different web sites for each of their brands - M&Ms, Uncle Ben's, Pedigree, Whiskas, etc. These all have their own web sites separate from the Mars Inc. The Mars Inc page does have a list of brands: http://www.mars.com/global/brands.aspx
Check Mintel Academic Reports for the market of the product line.
Check Passport for any market data or analysis of the market for the product line.
Check Nielsen Segmentation & Market Solutions for categories of owners or buyers. Sometimes the sheer numbers of potential customers in the market are helpful in this area.
You might want to each go to a local store to see product placement on the shelf (shelf space & height) and check the price. Get permission before taking a picture of the store display (you don't want to be suspected of corporate espionage). Go to specialty and general stores. If it is in a grocery store, try more than one chain in different economic neighborhoods.
Whiskas AND markets
Some brand names are regular words and can result in many false hits. For those do something to narrow it down:
Pedigree AND "dog food" AND markets
"Pedigree brand" AND markets
Collect advertisements on the web, print, and TV/cable. If you can get it, you can do radio, too. Not all stations have full recordings of specific broadcasts. Keep track of where & when you found the ad. Name the show or web site it was linked to.
Check for coupons. (Sunday &, locally, Wednesday are the big inserts & coupon days in newspapers, Tuesday is the coupon mailers)