I've seen and designed a few sets with these designed into them. Notably, Mogoholic on her priest's page has made excellent use of the various color variations. If you haven't seen her website, Mogoholic, you should check it out! I had a lovely screenshot of all three, albeit my Death Knight was wearing the Arachnidian Robes. :p
There are 3 available coloration options of these spider robes, and accompanying pieces. The 4th coloration, was in the Naxxramas database back when it first came out., but the top half, the Watcher's Tunic, is available from a quest in Outlands. Let's spin this shroud of webs, shall we?
Venomshroud & Hibernal
Both of these items are Bind-on-Equip (BoE), and readily found on most auction houses for less than a thousand gold. The chest and leg options from the tunic set can also be found for very little. Certainly the most affordable option, especially if you like green. Farming them is possible, but for the cost versus time it would be best to buy them.
These robes can be found on Kalimdor, mostly from mobs in Silithus, instances, and raids. Trying to farm it would be... inadvisable? Not advisable? Let's just say you would need to put quite a bit into the alter of RNG gods to find this as a drop for yourself. Watch the auction house, where they can drop below 5000 gold occasionally. Usually they are around the 15,000 gold region. Patience is key, either path you choose.
Chan's Imperial Robes
However, never fear! These robes are back in Warcraft! Acquiring the robes is straightforward, if not a little cumbersome. You will need to go to Old Blasted Lands by speaking to Zidormi at the border between Swamp of Sorrows and Blasted Lands. There, you will find Flawless Draenethyst Sphere items that drop from the mobs in the zone. Completing the quest Kum'isha's Endeavors will give you an Emerald Encrusted Chest. Despite the list not showing it, the robes do in fact drop from that chest. Watching the auction house could pay off, where they show up for anywhere from 5,000 gold to 100,000 gold. Quite a swing, I know. :p
MOG ALL THE THINGS! :p