If you aren't lucky enough to have some fresh crawfish available, you can find frozen tails in a lot of better supermarkets or seafood markets. They generally come in 1 pound packages. I highly recommend that you use Louisiana crawfish, but read the labels-often times what you find is crawfish from China. To me, its the fat in the Louisiana crawfish that makes them sweet. The Chinese crawfish taste bitter to me and not sweet like the ones from the Bayou State. If you have to use Chinese or anything except Louisiana, rinse them well prior to using, let them drain, then pat them dry with paper towels. You can also buy Louisiana crawfish on line at sites like Cajun Grocer or Tony's Seafood (no affiliation with either company).
If you can't find commercial seafood stock in the grocery, you can substitute chicken stock, but if you're lucky enough to have some leftover crawfish shells and heads, the best etouffee will be made with homemade crawfish stock.
Etouffee is also very good made with with shrimp or chicken.