The Sidereal Essence vendor in Wrath of the Lich King Classic is a new addition to the old expansion. This currency allows you to buy 10-man Ulduar gear, both normal and hard mode versions, giving you a guaranteed way to get the loot you want without having to rely on RNG from boss drops.
You won't just be able to farm Sidereal Essence endlessly though, otherwise, what's the point in doing the raid? The amount of currency you can get is limited, so instead, think of it as an option to fall back on, and one which lets you target the specific item or items you need. Handy if you're down on your luck or can't make a raid or two. With that in mind, here's where to find the WotLK Classic Sidereal Essence vendor, as well as how to earn the currency.
Wrath Classic Sidereal Essence vendor location
The Sidereal Essence vendor, also known as the Animated Constellation, can be found in Dalaran. For Horde players, head to Sunreaver's Sanctuary to the north of Krasus' Landing, while Alliance will find this vendor at the Silver Enclave—check the map above if you're unsure. Here you can buy gear that drops from the Ulduar 10-man raid, and you'll need to spend anywhere from 15 to 38 Sidereal Essence, depending on the item you're after.
You'll earn one Sidereal Essence from the final boss of a Titan Rune Dungeon, so in theory, you could get 12 in one day, assuming you have the time to complete that many dungeons. If there's a piece of Ulduar loot you can't live without, it's a viable way to make sure you get it, otherwise it's a nice bonus now and again for clearing content you'd probably be doing anyway.