Where to find level 6 territory?

This will depend on your doomsday season but in general the outer ring / area of the world map will contain plots from 1-5, the second ring in will contain plots form 5-9, the third ring has plots from 9-12 then finally the center area has 12-16. Occupy plots and move towards the center of the map to find higher levels. And check if any of your alliance members already have plots in the area you want to be in.