Though related, this is no longer true. You can now purchase a lvl 90 character boost. Some power levelers have hoped for a day when you could use real money to buy individual items in-game, some times called "micro-transactions." At a Game Developers Conference 2008, Rob Pardo gave a strong indication, Blizzard would not ever go this route with World of Warcraft.
As the first major content release of Battle for Azeroth, patch 8.1: Tides of Vengeance is adding a number of new ways to gear up, as well as increasing the item level of rewards from existing activities. Tides of Vengeance goes live on December 11th and will add most of the new content, but Mythic+ and PvP Season 2 will not start until the new raid, Battle of Dazar'alor, releases in January, at which time the item level thresholds will increase to the new Titanforge cap of 425.
Heirlooms are special pieces of armor that level up with your character, so they have stats that are always respectable for your character’s level (up to level 110). Heirlooms that fit the head, shoulder, chest, legs, and back armor slot offer an experience bonus. The total bonus is 45 percent if you have them all. It’s also possible to acquire a ring that adds another 5 percent, but you must win a fishing contest acquire it, and that can take some time.