You buy up to mageweave in the same place you buy the book that lets you go to 225 skill. I don't kno where that is for alliance, for horde its that swamp off the barrens, forget name. To get the heavy mageweave, runecloth and heavy runecloth you return to who gives you the quest that lets you go to 300 skill. Again I don't know who that is for alliance, but for horde its the trauma doctor in hillsbrad foothills city (gotta be level 40).
He will train them for free but they won't even be visible unless you have the skill to get them, for example: talk to him at 225 skill and it will appear that there are no skills he can teach, talk to him at 260 and he'll say I can teach you this.