The other week I was shopping at Wally-World (Wal-Mart) and found in their grocery section a bag of greens called "Green Trio". It's a bag full of cut collard greens, turnip greens, mustard greens, and kale. (I know that's four and it's called "trio" but apparently they rotate what greens are used throughout the season).
That's a pretty decent blend of greens, except for the kale but the bags I saw were mostly collard greens, and it was only $3 or so. The collard greens still have some stems, so you may have to cut them for smaller iguanas.