I'd also been looking for one without success, so I finally tried making one.
http://pastebin.com/4YEKNfZm
Notes:
-You need to create the floor of the 1st level manually (or use a different turtle program to create a platform).
-You'll need to place the 8 water source blocks per level manually, break out the center block for mobs to fall through, and would also recommend manually placing a half slab in the middle of each pad to prevent spider spawns.
-Turtle will build forward & to the right of starting spot, placing blocks the level below it. Total size is 19x19x5. Turtle will return to starting area for the next level up each time. Its inventory should be refilled, then program run again for additional levels.
-Coal goes in turtle's 1st slot, blocks in the others. Takes about 8 stacks/level.
