Forget about the pellets.
You will be far better off by buying a volumetric powder measure of up to 120 grains, a horn or flask (the Treso flask with built in valve and measure), and a can of triple 7. Your accuracy will improve because you can adjust the load for optimum performance. The powder charge will be easier to ignite. Your cost per shot will be less.
By the way your best performance will be found using real black powder.