So I get that theres a 1/4" aux on the back of both monitors, plug the left into one monitor and the right into the other, but do I plug them into the "line output" (two 1/4" slots on the back of interface) or do I buy 1 cord that has a 1/4" on one side and two left and right 1/4" jacks on the other end to plug them into the "headphones" slot on the front of my interface?

You need TWO cables which have 1/4" MONO TRS jacks (one on each end). Left and right need their independent cables.

You don't need expensive cables. I wouldn't buy the CHEAPEST, to avoid shitty connectors and materials, but it doesn't have to be expensive either.

I searched a bit and found these, they look good. You need to get two of these: https://www.amazon.com/Monoprice-Premier-Right-Angle-16AWG/dp/B00AJHCWH8/

https://www.amazon.com/UGREEN-Premium-Unbalanced-Instrument-Housing/dp/B00Y44AAQW/

https://www.amazon.com/Gold-Plated-Copper-Shell-Heavy-Duty-Housing-Amplifiers/dp/B073XY1KY3/