Michael Mountford had denied the charge which followed an investigation into the death of 18-year-old festival-goer Christian Pay, who became ill at the event in July 2015.
The 42-year-old, from Oak Drive in Denton, Manchester, was found guilty by a jury at Carlisle Crown Court.
He will be sentenced alongside two other men on 1 August.
Simon Chapman, 21 and from Millom, and Luke Fraser, also 21 and from Manchester, had previously admitted conspiracy to supply Class A drugs.
A man has been convicted of plotting to supply Class A drugs at the Kendal Calling music festival in Cumbria.