There are some low cost pens that come with a Flex nib you might be able to rob the nib or section from. Noodler has one with a cool piston filling mechanism (the Ahab) I've been wanting to adapt to a custom pen.
(Edit). The Goulet Pen Co. has the Stainless Steel Noodler Flex nib for sale for $6, #6 size.
Thank y'all. I used to make rollerball and pencil sets as a hobbyist several decades ago, and wanted to get back into turning and do some fountain pens.