Is there somewhere I can purchase replacement nibs for a recent model fountain pen, such as my Sheaffer Prelude, my Parker Sonnet or my Waterman Phileas? All three of those pens have "medium" nibs, and I am finding that I prefer the more precise line of a "fine" nib. Do any of the online stores carry nib assemblies for these (relatively) inexpensive modesl? Can I purchase replacement nibs from the manufacturers themselves?
(BTW, all of these pens are used, so they don't really qualifiy for a nib "swap" from the company.)
Thanks in advance for any advice that you all may have.

TMann