It depends on whether or not Dell used non-standard wiring for their case fans to make you buy their fans or re-wire any fans you want to use. Are you talking 120mm fans, 92mm, 80mm? What are the dimensions of the fans you want to replace? Connectors available to power the fans? (4-pin molex from the PSU, standard 3-pin or 4-pin mobo headers)
Who told you that you could only buy replacement fans from Dell?
Edit: Nice thread-jacking