Funding office space? The government must have spare office space somewhere in the United States. Funding thousands of square feet in expensive cities is not required in the virtual world of the 21st century. How about one shared office to support ex presidents on a military base? Mail processing and scheduling of appointments can be handled for all of them by a shared pool of administrative help. Pay for each ex president to have one confidential secretary for the first 10 years after leaving office. All other services to be performed by the shared pool. After 10 years the ex president should be fully retired or able to pay for his/her own administrative support.
As for Secret Service protection, that is a cost we will have to bear for the president and his spouse for their lifetimes. Unfortunately we live in a world where they are targets of terrorist and psychopaths.
IF THEY'RE WEALTHY.
Former Presidents can and DO command high speaking fees.
They can pay for their own stuff.
President Trump has been a beacon shining the way for Presidents to act regarding what they take from we, the people.