You can try your luck finding working girls in Santa Marta from these places:
Olds Bar, Carrer 2, between Calle 18 and Calle 19. 8000 peso for a beer. Over 20 girls. 200,000 pesos for take-out girls.
Renobar (CLOSED), on Calle 10, near the corner with Carrera 3 (use google maps for reference)...