Our favorite gyros place is the Gyros King on the southeast corner of Ogden Avenue and Main Street in Downers Grove. I'm sure they will carve and sell gyros in bulk; don't know about pita, but I'm sure you could work something out.
I would make my own tzatsiki; the commercial stuff never has enough garlic for me, and it's usually a little skimpy on the take-out sandwich. Also I like more onions than they usually provide. For all that, their gyros sandwich is the best I've had around the western suburbs.
Interesting note: in Chicago, it's pronounced HEE-ros, but in Pittsburgh, it's called GUY-rose (hard G.) No idea why the difference.
Mike, the honorary Greek.
In the Greek language, there is no word "Greek." They are "Hellenes."
Suburban gourmand