Honeybee --
Given the nature of the type of store you are dealing with here I would leave PETA out of the mix. The hunting thing will be the bigger issue and chain store like this will be a good target for them. Your poor little slider friend's plight will fall right into the toilet. Afterall -- you want to help the turtles not shut down a hunting goods store.
Check out Austin's turtle page on the that site has caresheets for all types of turtles.
BTW -- how big is the set up these animals are in and is it clean and filtered. If it looks good in these aspects how about a different tactic? Go in an complement the positive aspects of the set up and commend whoever is in charge of these animals. Then lead in with things look nice but if you just had XYZ for the basking area etc. and Say that you would be willing to help set it up the right way.
I have found that sometimes "embracing the bad guy", if you will, gets you further. Don't say YOU ARE BAD!! Say this is okay but it would be sooo much better if you did this. Let me help you etc.!!
When you are making alot of noise and only being further ignored you have to try another approach. This could work well for you and also educate these folks on the proper care of the animals.
Realistically -- if these animals have a clean environment that is suitable in all aspects except for basking they are far better off than many sliders out there. (I am not being an advocate of improper care believe me but when you look at the whole picture at large 3/4 of loaf is better than a few crumbs.) Of course I don't know what the rest of the conditions are actually like.
Good luck and keep us posted.