Browse the ASA site info and statistics section and follow some of the links and try contacting them for info and referrals of people and orgs to contact.
There are some market research firms out there that have the data, and they would know who to contact.
American Sportfishing Association
Another source is Fishing Tackle Retailer, a magazine that includes all kinds of fishing, but might be able to cut some data for you, or give you some leads on who else to go to.
ESPN - February 2009
But ultimately, I think you might be best off trying to design a survey and send it out to a bunch of retailers in your category, with the offer to share a summary of your aggregate results with them, but not identifying responses from individual stores to protect their info. Such a report might be useful for shops to benchmark their results, and should be pitched that way. They should be designed to be easy to fill out but give you the data you need. For the design of the data collection, work backwards from the results you’re trying to get in terms of data displays, to get the questions to ask. You want the actual choices on the data collection to be unambiguous and exclusive ( ie not “Are your annual sales 2-3 million or 3-5 million?” etc where someone with 3 million in sales could show up in two places.)
You might also be able to hunt up potential targets on Hoovers or Dun and Bradstreet using SIC 5941
Description for 5941: Sporting Goods Stores and Bicycle Shops
You may want to talk to the research librarian at the business school to see what subscriptions they might have that could help you drill down to get the info you seek.
A simple google of “fly rod reel line accessories fly tying material” will turn up a gazillion online retailers.
As I’m sure you’re aware there has been massive consolidation in the industry, with large retailers with their own distribution chain carving up a greater share of total sales, and a very difficult competitive environment for small-medium sized shops and larger independents.