Can anyone tell me where I might find an index to help me find a topic in Mishna Berura (preferably in English)?
There is an index called Yad Yisrael
Dirshu has an excellent Hebrew index on all 6 volumes of Mishna Brura, in one volume covering 6,000 categories and 30,000 sub-categories over 600 pages. You can see sample pages here. Although it is in Hebrew there are a few dozen pages in English at the end with words translated from modern Hebrew.
See a background article on the publication of this volume here.