|Jira Agile 6.3
As of JIRA Agile v. 6.1.3 you can search for a Sprint by name using JQL: Sprint = "MySprintName".
In versions prior to that you must search by Sprint ID number. More details on valid search syntax in JQL can be found here - Advanced Searching
Follow these steps to retrieve the Sprint number that a given issue is in:
Alternatively, to get the Sprint ID copy and paste the following in to the browser's developer tools console while you're on the Work mode of a Rapid Board: