Welcome to Tomatoville Nathalie! : )
You might want to ready this thread.
http://tomatoville.com/showthread.php?t=36069
There's lots of excellent breeders and sellers here on TV. You can't go wrong with seed from them.
If your looking for specific seed type you might go down to the forum called Wanted and post your request. You need to make sure you are logged in to view and post in it.