Where are the Catholics who believe in this
form of the earth?
There used to be a small vocal group of flat earth proponents on the forum who posted on the topic so much that it was derailing other topics and disrupting the forum. Matthew, the forum owner, created this sub-forum for the topic which made it easy for those with no interest in it to ignore it. After a while, even the flat earthers seem to have lost interest.
There were never very many of them on this forum to start with. It is not really a position that one would expect traditional Catholics to adopt since, for over a thousand years, the traditional position among Catholics has been to think that the earth is a sphere. Early in Church history, it was a matter of disagreement among the Church Fathers, but over time they reached a consensus. By the time St. Bede taught spherical earth around 700AD there was complete acceptance (at least among anyone for whom we have a record of their views.)
There are websites that deceptively present quotes from the Church Fathers to imply that they as a group believed in flat earth. If you have accepted f.e. because you were tricked by this, please rethink your position. There was no unanimity among the Fathers on this. This means their beliefs are their private opinions with no authority for future Catholics. And after the patristic period, there is virtually no evidence of any Catholics who believed in flat earth until the recent revival by neo-pagans and fundamentalist Protestants.