When you’re in-world, the easiest way to find new locations on the Mainland is through going to Land Sales on the (Firestorm) search tab. Then, choose “mainland sales”, an area bigger than something, and tab search. If you filter to show only Adult mainland, the only continent you’ll find is Zindra (any adult parcel in Mainland search will take you to the elusive Zindra.)
Where Am I -map
This map shows continents by shape, to be used as a pocket guide as to where you are if you know you’re on some mainland continent, but want to know which one. For instance, you go through a land search, and you want to buy land on Heterocera, but you don’t know where each plot is located. Use CTRL+M to open the map, zoom out as far as you can see the shape of the continent and compare to this image.
Visit mainland continents on Second Life
Linden Labs maintains a list of rezzing zones around mainland continents. They’re a great starting point to go explore all the weird and wonderful on mainland parcels.