How to select resistor with specific package size
The "proper way" is to make the footprint yourself. That way, you can be sure the footprint is correct (and, when it comes time to make a more complex footprint, you will already be familiar with the process).

If that sounds unpleasant, you can simply Google "1812 resistor eagle" and hope the person who made whichever one you find knew what they were doing, but understand that making your own footprints is typically a necessity of board design (unless you work in a company that has engineers specifically for that sort of thing).