My mom and I took the Ninja Turtle pattern 5143 Butterwick which is hard to find and we modified it to make our Ninja Turtles costumes. We chose not to make a big ninja turtle head, we used masks only and we made the turtle shell three dimensional by stuffing section of shell with fiberfill stuffing.
We chose a fabric to replicate a turtle shell. We took faux brown leather for the straps on the legs and chest. We used felt with fiberfill. We also made the feet smaller so the kids would not trip. Also a strong fabric or sole is recommended to keep turtle feet from tearing. We used a lot of Velcro.
At the time the new Ninja Turtles movie came out so they had the swords and nun chucks accessories. My daughter wanted to be a turtle with her brother and cousin so we made her "madonna"tello with a pink mask and a purse as a weapon.
Total cost and constant visits to fabric stores was about $150 plus accessories. They took about a month each to make because we did it in our spare time. The kids loved it and that's what made it well worth the sweat and tears put into these costumes.