Traditional penpals (i.e. using snail mail) are largely a thing of the past these days, as most contact is online (either directly by email or through social media sites, such as Facebook).
However this site might be a good place to start looking for pen friends (irrespective of the potential means of contact):
https://www.penpalworld.com/
(Most other penpal sites seem to be based either around learning other languages or finding love interest. That one might be your best bet for more general contact).
Use the links here to view the websites of newspapers in Devon, to see if they look like the type of papers/websites that publish your request:
http://www.newspapersintheuk.com/england/south_west_england/devon.php
(The corresponding link for Cornwall is here:
http://www.newspapersintheuk.com/england/south_west_england/cornwall.php )
Most sites will have an email address for the paper and/or a postal address.