Corynebacterium diphtheriae can be grown within 6-8 hours on:
On Loeffler’s serum slope, C. diphtheriae grows after 4–6 hours at 37°C producing small, circular, and white opaque colonies. 
If no colonies are seen, the medium is reincubated further for 24 hours.

Parija SC. Textbook of Microbiology & Immunology. Elsevier Health Sciences; 2014. Page:218
Potassium tellurite media with iron
McConkey’s agar
Dorset egg medium
Loeffler’s serum slope