How long ago was your certificate issued ? IP's are not required to keep records longer than 6 or 7 years, but there might be some archival records with the provider or with the Insolvency Service itself.
Your first port of call should be your former IP. Also scour your old emails as some providers sent copies out by email as well as hard copy.
My opinions are merely that .. opinions based on experience. Always seek professional advice.
IVA Completed 23rd July 2013 .... C.C. 10th January 2014