I don't think you will have any problem finding shooting supplies for the .54 Cal, and I don't think there is much difference in the cost of components between .54 vs .50. The larger projectiles may be slightly higher cost. One thing you do need to find out is what rate of twist your rifle is. Some of the Lyman Great Plains rifles are 1" in 66" which makes them only suitable for round ball (patched ball). If it is a fast twist, it may only be suitable for conical or sabot bullets. I" in 48" twist may shoot either the round ball or conical, although my rifle likes the patched round ball.
By the way, you have a really nice rifle at a fair market price. These rifles are made by InvestArm from Italy and are of a very high quality for a mass-produced rifle. When I first started shooting black powder rifles in Pennsylvania, I went to a store that specialized in only black powder rifles, both commercial and custom made, and asked them what I should start with as a first time black powder shooter. This place, known as Dixon's Muzzle Loaders, recommended either a Lyman Great Plains or a Lyman Trade Rifle in either .50 or .54 Cal. I chose the Lyman Trade Rifle in .50 Cal which was a bit shorter and had a 1" in 48" twist barrel making it an all around barrel for round ball or conical bullets. I still have it today.
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