Blizzard has changed the item that is required to change the talents and glyphs.
To modify any of these, the talents or the glyphs, you will require an item called “Tome of the Clear Mind”.
Here is a quick answer for your question.
WoW Mop where to buy Tome of the Clear Mind?
Tome of the Clear Mind can be found at the “Alchemy Supplies and Reageants” in Stormwind.
Very easy to get and to find the location.
The “Alchemy Supplies and Reageants” building in Stormwind is actually the old building from where you usually bought class items in WoW Vanilla.
The building is located in the Trade District.
For the Horde players, the items should be available also at the “Alchemy Supplies and Reageants” vendor in Orgrimmar.
Some Mists of Pandaria mounts might also sell them within their vendors, like the big woolly mammoth.
Except these places, you can get Tome of the Clear Mind from Jeeves (engineering profession item).
In Pandaria, if you are around a place with many vendors, one should sell the item for sure.
Tome of the Clear Mind costs 35 silvers, for those that are in a level 25 guild.
You should buy some of these items, around 10-20, since they are required to change both talents and glyphs and since these are situational, stacking these tomes is a great idea in order for their requirement to not bother and interfere with your gaming experience.