I heard you can get extra skill points (besides the ones you get from leveling) by doing certain missions or something. Does anyone know how to get these extra skill points or where to find the missions? Any help would be appreciated, thanks.
You get these through doing client orders for "class trainers", which are class specific characters you see scattered through the gate area. You need to do these as the class you want the points for. After doing certain orders for them, they will have a mission that gives +5 skill points. After completing it, you get another order for 5 more, and lastly, you will be getting orders to give them class x cubes, which are cubes you get every time you "level up" a class after level cap. You get these missions one by one until you reach 10. So in total, each class specific trainer (or most of them, not sure) should give you 20 SP.
Edit: Correct quantity noted by @Jamesmor , who has done the content already.
Look for the class specific npc on the gate. They are all around the skills area. You have to complete a few of the orders for them to pop up.
@ZorokiHanuke you are wrong about the quantity, they give only 14 SP
these are the orders you need to do
all trainers give the exact same orders with the same objective, the first one will unlock when you get access to hard difficulty, the second to very hard difficulty and the last 4 will unlock when you reach max level however only the first supremacy will unlock.
supremacy requires class Ex-cubes that you get when you reach a certain exp at max level
@Jamesmor How many class x cube orders they give? I haven't done it myself but I kept hearing it was 10. I did the initial 2 orders for 5 points each, so I know that we can easily get that much so long as your character can do certain content up to hard difficulty. Anyway. I edited my other post so they refer to yours for the correct number.
@ZorokiHanuke first order requires 1 ex-cube, second 2 ex-cubes, third 3 ex-cubes and fourth 4 ex-cubes so 10 ex-cubes in total however each order only gives you 1 SP and you get more than 1 ex-cube when you reach the exp required for then
@Jamesmor Alright. As I said, edited the older post to name you so they check your post for the correct number. Thank you.
Thanks for the info guys! Much appreciated 
Hopefully some others can read this thread and learn something as I did
Since it wasn't mentioned, the four SP you can get from your class trainer for the Ex-Cubes will require roughly 20 Million XP for the requisite 10 cubes needed for your four skill points (or the equivalent of two additional levels past level 80).
@Blade-Syphon The Class Excube to EXP ratio is actually better than it was at lv75 since you get 5 Class Excubes per over-level at lv80. In fact, as the level cap gets increased, you need less and less total EXP for each Cube and you get more and more per over-level.