@luke , Luke Soules , If you have an old caster assy. take it with you to hardware/surplus store to compare with what they have in stock as they may have a replacement. Posted a few links below for replacement stem casters. Good luck.
I hope this helped you out, if so let me know by pressing the helpful button.
http://www.homedepot.com/b/Search/N-5yc1vZc29f/Ntk-Extended/Ntt-caster?Ntx=mode+matchpartialmax&NCNI-5
https://casterconnection.com/casters/stem-mount.html?p=1
http://www.faultlesscaster.com/page.php?menu_id=11