Two companies which offer a variety of board game-sized sleeves are Mayday Games and Fantasy Flight Games. FFG's better organized, but they only offer what Mayday would call "premium" sleeves (the thickness you're probably used to with e.g. magic sleeves), which run a bit over $2 for a pack of 50. Mayday also offers "penny" sleeves in packs of 100 for about the same price.
Fantasy Flight has a document to help you figure out what sleeves to use for their own games; click the link for the Fantasy Flight Sleeving Guide on this page. Citadels requires 3 packs of the green sleeves ("standard american board game" I believe?). FFG's green sleeves equate to Mayday's chimera sleeves.