You can find stat blocks and lore in a bunch of modules throughout D&D.
1e has versions of Mordenkainen in the
Rogues Gallery, in the parody
Castle Greyhawk module, and in
Mordenkainen's Fantastic Adventure where you can play Mordenkainen, Bigby, Lord Yrag, and Riggby as pregens. The rogue's gallery also has Bigby, Erac's Cousin, Riggby, Robilar, Serten, and Tenser. Stats vary wildly, Modenkainen is 12th level in the Fantastic Adventure, 14th in the Rogue's Gallery, and 29th in Castle Greyhawk. Bigby is 10th level in Fantastic Adventure, you need to be 18th level in 1e to cast the 9th level Bigby's Crushing Hand spell, and at 10th he cannot even cast the 6th level Bigby's Forceful Hand spell.
You can find stat blocks for most of the Circle of Eight (not Mordenkainen though) in the 2e module
Vecna Lives! where you also get to play them (Bigby, Drawmij, Nystul, Otiluke, Otto, Rary, Tenser).
2e City of Greyhawk has six pages covering stat blocks and paragraphs of lore on the Circle Wizards pre-Greyhawk Wars.
2e Rary the Traitor has a bunch more to say on Rary and Lord Robilar post-Greyhawk Wars.
3.5e's Expedition to the Ruins of Castle Greyhawk has some lore on Mordenkainen and Robilar (and Iggwilv) as well.