Where can I find information about upcoming special issues in a field of study?
In addition to the two sources mentioned by William, I can add professional societies or their equivalent. It is usually these that organize meetings that result in special issues. It is rare (may differ between disciplines) that journals themselves, unless they somehow specialize, provide open special issues around a theme. Usually, it is then by invitation, or papers emanating from a workshop or symposia etc.

Hence, looking for especial issues may be a dead end and you should be looking for activities that lead to such issues.

As a final note, the term special issue signals an unregular issue in a scientific journal open for submission of manuscripts. Many conferences issue proceedings but then the prerequisite is usually attendance at the meeting and presentation of the paper there.