Creating a Power of Attorney: Power of Attorney : Deceased Father

My question involves estate proceedings in the state of: Texas

My father recently passed away and unfortunately we can't find the keys for his storage unit (Public Storage) and PO Box. We were informed that in addition to needing a death certificate (which we have) we would also need a power of attorney to give us access. I live in Oregon while my mom is still in Texas. What do we need to do in order to get a power of attorney?

Thanks!

Creating a Power of Attorney: Power of Attorney : Deceased Father