I've gotten them from Sal's Seafood 3 times in the past 2 weeks and they are INCREDIBLE! It's rare that I find a boiled seafood place that does everything right. They do. The crawfish are perfectly seasoned and cooked, as are the potatoes and corn.
1512 Barataria Blvd, Marrero, LA
Yes, the Westwego Seafood Market. I've had crawfish from there twice now so far this year; we did a boil yesterday and they were a little bigger than two weeks ago. $100/sack, which is $2.50/lb. They'll continue to get cheaper though, if you wait a few more weeks, but that's an ok price as far as I'm concerned.
From the Go Green Nola website:
Westwego Seafood Market
Daily (More vendors on weekend)
100 Westbank Expressway
This is an open market where you can get fresh seafood from the fisherman. Best to visit on the weekends as that's when most vendors are there