Turtle soup is a traditional Cajun dish. To be honest I'm not sure where you purchase turtles, but if you go canoeing you can find hundreds on logs in the river. You can also catch them just like you'd catch a fish--string a line with chunks of meat on bulky hooks near the banks of the river. I'll discuss how to remove the shell and all.
First you need to get the shell off the turtle and get it soft enough to chop. Be humane and kill it first. Then boil the turtle for about a half hour. Take it out and you should easily be able to pry off the shell. Remove the organs and chop the meat into bite-sized pieces. Saute them and the onion for about 15 minutes, until the turtle is good and brown. Stir in the remaining dry ingredients and maybe 2 liters of water. Bring this mix to a boil, and simmer for an hour. Stir in the remaining ingredients, heat for a minute or so, and serve. Enjoy!