I like to do missions, but like to plan ahead. How can I find missions in areas not local to me?
I think that in these moment there's no way to see more mission far away from yor location.
If you plan for example go to a city or some special place, you can talk with the players of the zone, ask for mission that can be done, and if they can give you the locations of the portals of the mission. (Ask the local player always is a good way to maximize "benefits")
In the last versions of the game, you can find wich mission have one portal. To see it, you have to enter in the portal info, and you will see the new button. This will show you a list of it.
Also in www.ingress.com/intel you will find the missions. Thanks to this change you will be able to go to the city you want, open the mission list, and select what you fit more to you.
Button in the intel map (browser)
List how is seen: (from Utrech as example)
You could go to http://iitc.caconym.org/mission/ but missions authors have to give authorization to publish their missions on this map.
Currently You can use https://ingressmosaik.com This site specially for Banner Missions. You can search them By your country. Also this sites provide plenty of valuable data such as Mission type (hack only, capture or upgrade etc), Number of unique portals etc