You can call T-Mobile Customer care, tell them that you'll be roaming
internationally, and ask them. I haven't tried it via phone, but they
used to have a relatively liberal policy in regard to giving out
unlock codes. In some cases, you didn't even have to be a customer for
a year before you could get one.

Until a couple of years ago, there was a special e-mail address that
was used for the processing of unlock requests, but that e-mail is no
longer accepted and everything has to be done via phone.

If they won't do it for you, a lot of phone dealers (e.g. not company
stores) do phone unlocking in major cities.