It turns out that d,l-limonene can be prepared via pyrolysis of used tires.
Details of the method used for this purpose and the temperature at which d,l-limonene cracks to isoprene can be found in the following publication:
Title: "Production of dl-limonene by vacuum pyrolysis of used tires"
Authors: Hooshang Pakdela, Dana Magdalena Panteaa, Christian Roy
Journal of Analytical and Applied Pyrolysis
Volume 57, Issue 1, January 2001, Pages 91–107