A front derailer designed for 48/36/26 or 46/36/26 triples will best match the curvature. This includes most MTB front derailers from late 80s to early 90s. As others have said, though, curvature isn't that important to match perfectly.
But as long as you're using a bar-end front shifter, you don't need to worry about exact cable throw because the front shifter isn't indexed.
If you are switching to brake/shift levers then you'll need a road-group front derailer whose cable pull ratio matches the shifter.