I think all you really need is the place that you interned to send you the following on their letterhead.
[quote][NAME OF ORGANISATION]
[ADDRESS]
To whom it may concern
Date
Dear Madam / Sir
This letter certifies that [NAME] completed a [PERIOD] internship with us between [DATE] and [DATE].
During their internship, [NAME] worked on:
[li][practice area][/li]
[li][type of work completed / etc][/li]
Please do not hesitate to contact us in case of any queries.
Yours faithfully,
[INTERNSHIP SUPERVISOR NAME]
[/quote]
Of course, the organisation would want to tailor it towards their own sample, etc, and may want to add more personal things about how you conducted yourself etc, but that is more or less up to them.
Hope this helps.
Best wishes,
Kian