How to step down voltage from 208V to 110V

As someone who worked in the hosting/datacenter management world I beg of you,

Don't Do That!

If you require 110V power contact your facility/electrician and have a proper 110V feed run to your rack - most facilities are equipped to provide conditioned UPS/Generator backed power at 110V as well as 208V, and already have equipment in place for situations like yours.
If they don't already have a 110V supply available they'll be able to work with you to provide a stable solution for your power needs.

Adding your own step-down gear introduces a number of problems, not the least of which is added load (electrical, but probably more importantly thermal) for the datacenter infrastructure, and would almost certainly be frowned upon by the DC Operations team (If it isn't you may want to seriously consider new hosting arrangements).

I agree with voretaq7, just install a special 120V circuit (and associated rack PDU) for that load. Alternatively find a dual voltage power supply to replace each el-cheapo power brick you have.

Do NOT use a IEC C14 to NEMA 5-15R cable for that. It'll certainly make the copper connection, BUT it won't step down the voltage. Those cables should be banned, IMHO (but I keep one in my bag, just in case.) They are dangerous in the wrong hands as they are a fire hazard. (Since you are asking this question I assume you know those dangers, but I want others to read this.) Others: ALWAYS check the allowed input voltage. Plug in the wrong thing and you'll cause a fire. Which is NEVER good in a DC.