Must be a regional thing, cuz I know you know the answer to that question. Canning and Pickling Salt is merely salt with no iodine in it. Same as used for curing meats, although I see you guys use Tender-Quick and such. I mix my own Salts w/saltpeter, or nitrates. Canning salt is available at most grocery stores and Morton puts out one in the bag...should be next to the Tender-Quick.
For dry brining fish, the ratio is two parts salt to one part brown sugar. Then any amount or type of spice you want. I made a fillet for testing yesterday with Jamaican Jerk seasoning in the dry brine and it was wonderful! You don't have to brine for long...30 minutes to an hour to 8-12 hours. All depends on the depth of flavor and how salty you want the fish, though all the salt should be rinsed off prior to drying and smoking.
I use rock salt from the fish cannery for my wet brine. I have also ground it when I ran out of canning salt. This isn't road salt...it's food grade sea salt. Any salt, as long as it's not iodized, will work. Kosher, too.