anyone know a place to get some Pyrodex® .54 Caliber 50-Grain Pellets

[MagKarl]

Those pellets weren't made with your sidelock rifle in mind.  Ignition is generally better and more consistent with loose powder in traditional style muzzleloaders.  Pellets are designed for inlines.  Just something to think about, it's your call though.  I'd get a powder measure and a can of powder if I were you.  What is the twist in that rifle?