Essential publication on the ad industry.
Many media companies list their rates on their websites. Just google [company, e.g., Wall Street Journal] ad rate and you should get to their "media kit" quickly. For companies that don't disclose details online, SRDS Media Solutions (listed below) is the go-to database.
It's difficult to find free information on ad spending of a company, esp. by brand and medium (e.g., TV vs. Internet). The PSU Library cannot afford the comprehensive database that provides those detailed data; however, we did purchase the 2015 dataset (the AD$ Summary file listed below) for your reference.
A public company's 10-K report may mention expenditures on advertising. Figures are annual and cover the entire company, not particular brands.
Anecdotal information might have been mentioned in news articles. Search strategy: