How to Obtain
These scrolls are mainly obtained through items called '技能書卷抽獎券' (Skill Scroll Lottery Ticket), of which there are at least 10 at current. These are obtained from within various lucky bags, or combined by putting together a lot of skill scroll fragments (I think you need like 60 of these). The fragments are also from lucky bags iirc. A few scrolls are also available at Trader Girl, in exchange for large amounts of Red Diamonds.
Players will sell these scrolls in stalls for various prices depending on the general demand and usefulness of the scroll. They can range from like 10 million down to like 20k depending on the skill usefulness and the desperation of the seller.
Here's an example of one scroll; this is the Joan's Jive Lv12 scroll. All of these scrolls use the same icon of a blue scroll. If it looks like a piece of paper, it's not one of these scrolls (it's probably a Sorcerer scroll recipe in that case).
Name Format: 【Lv#Skill Name】 書卷
How to know if a scroll exists, and if it does, can you even use it?
The easiest way to tell if a scroll exists for your skill is to check your 'next level' skill information for skills that maxed out at 70. However, you must be using the Chinese magic file to do this.
The words in red which I have squared off in green are what you are looking for. This is what tells you that the skill requires a scroll to upgrade it to the next level. Also take care to see that the 等級需求:70, for some of these skills will still need a higher level to use them, even if they need a scroll. The obvious downside of this system is that even if you reach 75, you will still need these scrolls to unlock the higher skill ranks. I believe that some of the skills with higher level requirements aren't in game yet, eg the Scorching Scroll upgrade scroll requires Lv73, and I have never seen it.
Keep in mind that these scrolls only UNLOCK the skill level, you still need to have free skill points to actually get the skill.
At this time, these skills can only be upgraded by a single rank.
When subbing these skills on lower level jobs, the game takes the original level requirement of the skill (eg 71) when it calculates whether your damage or duration is penaltied or not. If you don't understand what i am talking about, you can read this article about the Sub Job Level Penalty (on Frost's forum).