A Drupal 8 migration process plugin to to allow for fetching data from a CSV.

ValueFromCsv.php

<?php

namespace Drupal\migrate_custom\Plugin\migrate\process;

use Drupal\migrate\ProcessPluginBase;
use Drupal\migrate\MigrateExecutableInterface;
use Drupal\migrate\Row;

/**
 * Load a CSV and return a col for a specific row.
 *
 * @MigrateProcessPlugin(
 *  id = "val_from_csv"
 * )
 *
 * val_from_csv will allow you to load a csv into a keyed array of arrays and
 * cache it.  You can specify the column that the key is in and the column you
 * want to return:
 *
 * @code
 * process:
 *   type:
 *   field_extra_value:
 *     plugin: val_from_csv
 *     source: my_key_in_the_csv
 *     file: public:path/to/data.csv
 *     # Optional.  Defaults to column 0
 *     key: int
 *     # Optional.  Defaults to column 1
 *     val: int
 *
 * @endcode
 */
class ValueFromCsv extends ProcessPluginBase {

  /**
   * {@inheritdoc}
   */
  public function transform($value, MigrateExecutableInterface $migrate_executable, Row $row, $destination_property) {
    // Which file are we reading from?
    $file = $this->configuration['file'];
    // What column is our key?
    if (!$key_col = $this->configuration['key']) {
      // Default to first col.
      $key_col = 0;
    }
    // What column are we returning?
    if (!$val_col = $this->configuration['val']) {
      // Default to the second col.
      $val_col = 1;
    }

    // Can we pull the data from a cache?
    $cid = 'migrate:csv:' . preg_replace("/[^a-z0-9.]+/i", "", $file);
    $data = NULL;
    if ($cache = \Drupal::cache()
      ->get($cid)) {
      $data = $cache->data;
    }
    else {
      // Load the file as a csv.
      $data = [];
      if (($handle = fopen($file, "r")) !== FALSE) {
        while (($row_data = fgetcsv($handle, 1000, ",")) !== FALSE) {
          $data[$row_data[$key_col]] = $row_data;
        }
        fclose($handle);
      }

      // Build the array as a keyed array.
      ksort($data);

      // Cache the $data.
      \Drupal::cache()
        ->set($cid, $data);
    }

    if (!empty($data[$value])) {
      // Return our val.
      return $data[$value][$val_col];
    }
    else {
      // No row for $value or the row doesn't have $val_col.
      return NULL;
    }
  }

}