There is a TON of information out there. If even one firm in an industry is public, you can look up their SEC filings at Edgar. You can then look up their web traffic at Alexa, and look up that of their competitors. Hand wave a ratio between the two,and assume that the competitors have the same ratio.
Google the business press for interviews with founders, and press releases.
Find industry wide statistics ("hunting is a 12 gazillion dollar per year industry").
Call up a wholesaler who sells to the competition and say that you're thinking of launching a firm, and you want to know the process to purchase from them, and what their maximum discount tier is. Chat with the new accounts rep and ask how many folks qualify for that top tier. If you're thinking about selling shovels online, and Shovel Wholesaler Inc. has a maximum discount tier of 40% off retail for $1 million/yr in purchases, and they let drop that "just two or three" retailers qualify for that tier, then you can figure out that shovels.com, allshovels.com, and ishovels.com (the three biggest retailers) likely come in at 1.5, 2, and 4 million per year (or something like that...if the top seller was a light year ahead, they would have negotiated a higher tier).
Although I've not used this approach, you can call up SCORE and see if anyone there has info.
Ping your network on LinkedIn for contacts with info.
I note that your example (gamefly, netflix) is one that I've actually done, because one of my two firms (SmartFlix) is in this space. Their numbers were easy to get from the SEC. I've got a spreadsheet comparing all of their financial ratios to ours (cost of customer acquisition, lifetime value, revenue, profit, revenue per employee, etc., etc., etc.).
Never before in the history of mankind has it been as easy to get info on competitors.
Put on your Sherlock Holmes hat, pop that pipe in, and get to work!