Burnham S20 nib replacement

[Sagebrush64]

Hi all, I was going through some of my pens that have been set aside awaiting repair and came across that pretty little Burnham B56 that is still missing her nib.

My question is - does anyone here have any idea where I might find a useable S20 nib or is there an alternative size nib that I can use?

Thanks in advance