Comparing my shoulder pad with a ready-made one. I have had to add many, many more layers to my pad in order to get the same height as would be achieved with wadding, but the overall pad is more structured and will hold its shape better.
I used the method in Couture Sewing Techniques (Schaeffer, 2011) and built them up by basting together layers of breast felt (cotton donnette would also be suitable) and staggering the layers. The shoulder pads are shaped on the stand with steam.
In this photo you can see just how many more layers I have been adding, and the effect this has on the shape.
Custom-made shoulder pads are also good as they can be adapted for the individual figure. My model is petite and slim, and does not have especially square or pronounced shoulders. This (as well as the look as the period) is why I have had to add quite so many more layers to my shoulder pads: additional structure is required to create the period silhouette.