I am a future beneficiary and heir of my fathers and mother's trust. How can I get a copy of his trust in Washington State?

not if you are a future or potential beneficiary.

Generally future beneficiaries have no right to receive a copy of a Will or Trust until such time as they are a present beneficiary of the Estate or Trust.

Trusts are, at their core, a contract between the Trustors and the terms of each Trust can vary widely. If the trust left you a specific bequest at your father's death, then you might have some rights, however until your mother passes away she may still have the right to revoke the Trust or change the distribution through a power of appointment.

The short answer is that you are not entitled to receive a copy of the instrument until you are a present beneficiary.