Here is Markandeya Purana unabridged in English.
And from here you can get it in Sanskrit-Hindi (Sanskrit original slokas with their Hindi translations).
From this page you can read it in unabridged English translated by Swami Vijñanananda.
Some other Puranas that have information on Goddess Durga.
As is well known Durga Puja is quite famous in Bengal. And here they follow mainly four Puranas for the worship procedures apart from following the Agama scriptures. And, they are:
1) Devi Bhagavata 2) Markandeya Purana 3) Kalika Purana and 4) Brinnandikeswara Purana.
Of which , the last two are considered as upapurans. So, apart from the Markandeya Purana and the Devi Bhagavatam, the two upapuranas i mentioned also contain a lot of information on Devi Durga but i think it is hard to find them online.