Please help me identify this part!

[GLT57]

What is this called, and where can I get one like it?

[davent]

Looks to be the nut for a closed type 1/4" jack, Rean being one brand of jack.



Rean NYS215

[MikeA]

Switchcraft and Neutrik also make them, I don't know if this part (ferrule or nut) is interchangeable.  You might want to buy one of each brand of jack and see if the nut from one of them fits yours.  Small Bear, Antique Electronic Supply, etc carry multiple brands.

[Phoenix]

Originals are made by Cliff, their S4 series. Rean (Neutrik), CHK and others make clones (though I've not seen a Switchcraft clone).
Note that the originals have a 3/8" BSF thread pitch (20 TPI), while some others have a different thread pitch, 3/8" UNF (24TPI) if I remember correctly, and are not interchangeable.

EDIT: and others still, (such as Rean) have a M9x0.75 pitch (roughly 34TPI).