Thank you. One reason I went with the Ahab was the flex nib. Are there other brands that offer a flex nib?
Flex is kind of a massive topic. Many people will swear up and down that no flex nib made in the last two or three decades (outside of dip pen nibs) can hold a candle to "vintage flex". Personally I've never tried a vintage flex nib so I have no frame of reference and no advice on how to get your hands on a vintage flex nib, but I do suspect that "vintage flex" might not be a good starting point for a new fountain pen user as it takes time to learn not to apply pressure when writing the way you have to with a ballpoint pen, and I've heard that it's easy to push a vintage nib too hard and end up damaging it without realising it. So I would recommend sticking to modern flex nibs and dip pens for a while.
As far as makers of modern flex nibs, I am personally aware of a few possibilities:
Noodlers: Noodler's pens have a reputation for requiring tinkering, making them not ideal as starter pens for new fountain pen users. Their flex nibs require more force to flex than most pens.
Fountain Pen Revolution (FPR): have a number of pens with flex nibs that are very similar to Noodler's flex nibs (I've heard some people say their #6 is a bit better but their #5.5 is a bit worse). The Himalaya has a similar filling system to the Ahab, but the Triveni uses Standard International cartridges and converters (or can be used as an eyedropper, as can the Himalaya) which means it is easier to use and probably less likely to get ink everywhere (but it might be harder to keep it from railroading; you might need to find really wet inks to use in it). Both these pens are made of ebonite, which is nice.
FPNibs: can modify a modern gold Jowo nib to be flexible. I've heard good things, but you'll be paying around 140 euros or more just for a nib (but many pens will be compatible with that nib so the idea is if you find a pen you really like you can get a flex nib made for it later).
Conklin: offer flex nibs on some of their pens such as the Duragraphs (prices in the $60 range I believe). I have heard mixed reports about these pens and nibs, but some are very pretty.
Pilot/Namiki Falcon & Elabo: somewhat expensive (pens start in the $150 range but they have gold nibs), my understanding is that they won't flex as far as a Noodlers or FPR nib but they do flex more easily (and are available in finer nibs, which means even if they don't flex as far you're starting from a thinner line to begin with so you still get good line variation overall).
Regalia does sell flex nibs, but I don't know how often they are available or how well they work.
I've heard that Jinhao has a gold nib available in some pens for a fairly low price (around $40 I think) that has some flex. I don't really know the details.
I believe that there are one or two other manufacturers making modern flex nibs that I can't remember off the top of my head. Another option is a dip pen; dip pen nibs can give you a lot of line variation, but they are untipped so they are not very smooth and they wear out and rust quickly; nibs can be had for around a dollar I guess and I've heard they last around a week of normal use.
There are also fountain pens that takes dip nibs. Desiderata Pens is one option, although I believe a Jinhao x450 or x750 will also take Zebra flex nibs without needing any modification (but railroading might be an issue), as will an Ahab with a little work (or so I've heard).
By the way, this is a little off topic, but the Noodlers and FPR flex nibs can be modified to require less work to flex if you are willing to take a dremel tool to your nib (luckily replacement nibs are not too expensive). Check out this thread: http://www.fountainp...se-my-flex-mod/
Just a disclaimer: I'm interested in flex, but I haven't done too much experimentation and I'm still learning; I wrote in this post about the options I've learned of on this forum, but I don't have experience with that many of them.