It would be a big task but late spring each year you could start listing all of the fireworks shows and July 4 activities (concerts etc) around the country - and run ads for the fireworks vendors who sell in states where it is legal to do so, like here in Florida (of course they can't sell by mail). If you become known people would submit their event info to you, making it eaiser to collect.
Adding a line of patriotic hats, pins, shirts etc is also a good idea. It is very seasonal but could be developed to be well worth the reg fee.
In this case the .info is a good alternative to the .com as you would be providing information on everything going on around the holiday.