I am a big fan of Fountain Pen Revolution's flex nibs. They write great with very little railroading.
FPR offers these #5 size flex nibs (as well as their medium, fine and italic nibs) on many of the pens that they sell. I use the flex nibs on a few of my Airmail/Wality pens and they work great with the stock ebonite feeds.
Located in the New Delhi region of India, FPR not only sources the most popular pens from India, they also sell pens and nibs that they design and are crafted by Indian artisans.
They ship direct from India for only $3 and it only takes about 2-3 weeks to receive your order. All their items are reasonably priced and you won't easily find them anywhere else. Highly recommended!