QUOTE(dsatco @ Feb 14 2008, 08:20 PM) [snapback]514662[/snapback]
The Pilot/Namiki Falcon will fit the bill. Read the reviews, but it's definitely a keeper for me.
You can get a Falcon from John Mottishaw at nibs.com modified to make it flexier for a total of $201. Of course, I love love love my Pilot Custom 742 with FA nib which I believe to have greater flex even than the modified Falcon.
You might also consider an Omas Emotica, which is more flexible than a stock Falcon, but according to John Mottishaw not as flexible as his modified Falcon.
Of course there are the vintage flex pens, which I have less experience with, but vintage Waterman's pens are well-reputed for their flex. My Esterbrook with the 9128 nib has a quite pleasing flex to it.
The Sonnet is a nice smooth writer and I'd recommend it on those merits, but it's not at all flexible, so I'd say not the pen for you.