Finally! The Thrift Savings Plan now allows you to reset your password online instead of via the mail

You May Now Reset Your TSP Password Online

We’re pleased to announce that you may now reset your password online. If you forget or lose your password, go to the My Account section of tsp.gov, click on “Forgot your password?,” and follow the prompts. If you know your password and want to change it, log into My Account, visit “Profile Settings,” and click on “Change Your Password.” You can also call the ThriftLine at 1-877-968-3778 and press option 3 to speak to a Participant Service Representative.
https://www.tsp.gov/whatsnew/Content/index.html#reset