Currently, Salesforce is not providing Developer Edition for Marketing Cloud. Unfortunately, there is no work around like trial orgs also.

Check the following link and vote for this:
https://success.salesforce.com/ideaView?id=08730000000l1yFAAQ

Once the above idea is delivered, we will get Developer Edition for Marketing Cloud. We can test the Marketing Cloud features from Salesforce.

Keep checking the idea link or click “Upvote” to get notified when the idea is delivered.