I was going to mention Elliot Landes then it dawned on me, he IS penmakers.com. But keep in mind that he has been "going out of business" for almost a year now! Have you some updated info that would indicate he is almost out of supplies?
I ordered several of his Mutschler "Reform" sections as well as the Schmidt sections, both of which screw into little brass sleeves if you don't want to thread.
Other than that, Arizona Silloute carries the Berea nibs and feed. I use those when I make my own sections. They are compatible with upgraded nibs that Lou and Anthony sell as well as others.
I'd be curious to see who else sells sections to the public in small quantities.