Depends on how the power is supplied to your racks. 208V is a three phase "Y" service. That's what will exist at your electrical service panel. Each leg is 120V to ground (neutral / white wire). The potential across any two legs is 208V. I'll guess your PDUs are 208V "single phase" which means two legs are used instead of all three. If there is an electrical junction box above/below the rack that the PDU derives its power from, look for a white neutral wire inside. From the white wire to any hot wire is 120V. An electrician could create a pigtail 120V drop from the junction box to the fan. If there is no neutral wire then the above won't work. Note: Even though the green equipment ground wire is "ground" do not use it in lieu of the white neutral wire. The green equipment ground should never be used as a current carrying conductor.
Rather than creating a seperate 120V drop for each rack it might be better to create several 120V circuits. Each circuit would have a receptacle or wiring point at multiple racks - the number depending on the current load of the fans. Those circuits could originate at your primary electrical panel or some subpanel placed closer to the point of usage.
You do not need a step down transformer.
