To find bitter oranges you need to hit up the Asian, European and Middle Eastern markets and ask for bitter, sour, bigarade orange or Seville oranges(might need a translator)... Or, head out to the nursery and look for C aurantium subsp. amara, C. aurantium subsp. bergamia, or
C. aurantium var. myrtifolia.
Dry in a food dehydrator, or go old school and dry em in the sun.