https://www.treasurydirect.gov/indiv/help/TDHelp/faq.htm#Ope...
> In order to open a TreasuryDirect account, an individual or entity account manager must have a valid Social Security Number (SSN), be 18 years of age or over, and be legally competent. An entity must have a valid SSN or Employer Identification Number (EIN). The account owner must also have a United States address of record and have an account at a U.S. depository financial institution that will accept debits and credits using the Automated Clearing House method of payment.
AFAIUI, getting an SSN from abroad as a non-US citizen/resident is not realistic.
>Who may own an I bond?
>Individuals
>Yes, if you have a Social Security Number and meet any one of these three conditions:
> United States citizen, whether you live in the U.S. or abroad
> United States resident
> Civilian employee of the United States, no matter where you live
>To buy and own an electronic I bond, you must first establish a TreasuryDirect account.
https://www.treasurydirect.gov/indiv/research/indepth/ibonds...