Sean Bradley is the only person who makes not only ODST armor, but any Halo Armor, in a Kit fashion. While there are multiple vendors of Helmets, right now Sean is the only person who produces a ready to assemble kit for the armor.
There might be folks who will take on a one off commission for you, but guaranteed it will cost more than Sean's Kit due to the unique and personalized nature of that route vs the more Standardized/Fast Produced ability of Sean's Vac Form molds.
Other than that, the option is to make it yourself. There are plenty of options to pursue this option, as their are multiple files available in the Amory for Pepakura or Foam construction, as well as 3D printing files being available.