Handpan shells, are the blank steel domes from which Handpan can be formed, and tuned. They can be hand-hammered into shape (using a mallet, or compressed air hammer) from sheet-steel, or formed using other methods, such as spinning, and deep-drawing. And in terms of what sellers are currently offering, some come completely blank, while others are now being sold with either the Ding (top nipple-like note), or the Gu (hole in the bottom), already in place.
As more sellers arrive on the market, prices are naturally beginning to fall with the increased availability. And a number of highly-regarded makers are known to have either experimented with, or use exclusively, Handpan shells produced by other makers.
As hinted at above, probably the most important thing to check for when looking to purchase pre-formed Handpan shells, is some kind of evidence that the shells are fit-for-purpose. While the qualifications required to produce good Handpan shells, are not necessarily the same as those required to become a good Handpan tuner, without being able to hear a Handpan that has actually been made with the shells in question, you can't really be sure, that the shells being sold are suitable for the construction of Handpan. And have the right qualities to sing.
The thickness of the metal, for example, is just one factor, that if too far off, could render a Handpan shell useless for the purposes of building Handpan.