Sherman makes some good stuff but some stuff made by these companies are better suited to rain gutters than rocker panels. Sherman makes a decent outer rocker panel for the 49 thru 51 Mercury/Baby Lincoln 2-door coupes but they have a lot higher price tag.
A good rule of thumb is, if it's priced under $300 then it is likely a rain gutter. Look at the Ford and Mercury cars of the era and see how closely they compare. If they are close, you may have a potential to find some used parts that can more easily be made to fit. If not, then it's time to fabricate them yourself or find someone who can.
I need to get an english wheel so I can form the compound curves it takes to make patches more easily. I have a tree stump with all sorts of mallets & hammers but it is a slow way to go. A good fabricator can make what ever you need but it won't be inexpensive for sure. If they have an English wheel, a pull max, a planishing hammer, and shrinker/stretcher machines then they can make the stuff faster than I can. I spent a lot of time at auto salvage yards to find sheet metal of a similar shape and gauge to start with.
