The best place is the one that's closest to you, but still offers decent rates. All hatcheries get the same birds, as there's only a couple places in the country breeding actual Cornish X's, so go with the most convenient for you, and birds that won't have to travel across the country.
I get mine from the farm store here and they buy them from Townline Hatchery. I raised 26 broilers last year and they all made it to butchering age. Had one crippled one. I just bought 25 more. The chicks are healthy, alert and active. Here's the link. Their price for 100 is really good too.
I just picked up 10 Cornish X at my local Farm and Home Supply (FH&S) for $1.19 each straight run.
Priced better than I could order directly myself and I saved on shipping. They allow for ordering so I can add more if I want. Works out great for me since these are my first meat birds.
I'm really excited to have my first meat birds, since I have had so much luck with our laying birds. I can't wait to get them big enough to harvest them. (Don't know if that's how ya'll refer to it, but I hunt and that's the politically correct way of saying kill something when you hunt these days)
My wife and I own a finch breeding business and have been keeping laying hens to provide eggs for our breeding operations, as well as feeding our family. The best eggs I have ever had I might add. We recently had to travel out and ended up eating at an IHOP. While the food was what one would expect when I dug into my eggs I was really let down. . . They really tasted bland. First thing I did when we got home was to make and eat a couple of eggs just to get IHOP eggs as far removed from memory as I could.
I hope that Cornish X meat birds cause the same problem when it comes to eating chicken meat.
Sorry If I went off topic a bit. Try your local farm supply.