We don't have good midrashim books here in our town.
Do we have the story of Purim with good midrashim in English on the net?
Chabad.org has a online (all English) "Megillah With In-Depth, Side by Side Commentary" which you may find useful.
I know I am late to the party but if anyone sees this before next Purim here is a good book that goes through the entire Purim story with the midrashim and really added to my understanding of the Purim story. (You can order this on most judaica stores websites)
Let my Nation Live
There is in Otzar Hachachma (first 40 pages are free, but the rest must be paid for).