What is the 3D printer filament (or pellet) most resistant to bending at high heats? I'm looking to 3D print a structure that won't deform in high heat, up to about 220&nbsp;&deg;C. The filament itself can be 3D printed all the way up to about 380&nbsp;&deg;C. 

PEI seems like it could be a viable option. I found some here. This PEI filament  specifies the glass transition temperature at 217&nbsp;&deg;C.

Would this filament work? Are there any other types of materials that would fulfill this engineering requirement?
Your expected operating temperature exceeds the glass transition temperature by 3&nbsp;&deg;C. This implies that the structure will become weak and can deform under load. 

Note that you cannot simply print PEI on a normal machine, it requires a special high temperature capable printer with hot end temperatures up to 400&nbsp;&deg;C and heated bed over 120&nbsp;&deg;C up to 160&nbsp;&deg;C, furthermore it will need a heated chamber (up to about 80&nbsp;&deg;C) which requires special care to cooling and placement of electronics and motors.

Not having specified what kind of structure you require, you could look into steel.