Migrate: How to query the data source from within a process plugin

I want to query the migration data source from within a process plugin. Something like

class MyProcessPlugin extends ProcessPluginBase {

  public function transform($value, MigrateExecutableInterface $migrate_executable, Row $row, $destination_property) {

    // Query the SOURCE database?
    $result = $sqlbase_object
      // query conditions

    // Other stuff.

    return $something;

It isn’t obvious in the docs how to go about doing this. I assume I need to create an instance of my SqlBase-derived class, but how?

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