You cast site-searching spells with the ones that have level 2, once you have researched the spells and have the gems. You get the gems for that from the manual sitesearching performed by your first one or two Mot5E.
Every path has more 1st-level sites than sites of higher level, and more of those 1st-level sites are common. There are very few common level 2 or 3 sites. Check
the sitesearching thread for more detail.