Usajobs! Is your best bet! Unless, you know the supervisor and SES for that specific office who have authority to laterally transfer you in that specific office where you would like to work. You will also need to know the 2 budget/payroll officers and CFOs too.
You need to ask both the new office and old office in writing if anyone would be willing to pay for your relocation expenses.
Do be scared! Be empowered! If you don’t ask in writing directly, you will be blown off! You have to help yourself here.
A big plus for you relevant to your new office, you have a loaded FTE because you now become free labor to accomplish the mission for the new office. Your current agency can pay for a period of time, usually pending the duration of the appropriation, to work over in the new office. So, your new office can either choose to help you move with the savings or just work you for free.
Usually, you can call the directors (SESers and Supervisor) directly whom you would like to work for and ask if he/she would be willing to take you on! It is that simple! You can usually encourage and leverage this relocation through a loaded FTE from your old agency.
It is not easy! But, I have seem this been made to happen several time over the last 17yrs. Good luck!