"...on ebay..." E-bay is an evil, rabidly, anti-firearm ownership empire that financially supports the people who think you should not be allowed to own any kind of firearm.
"...sorry if this is a stupid question..." The only stupid question is the one not asked.
Gunparts lists Krag rear sights as sold out, but they ask $139.30 for 'em when they have 'em. Same rear sight on either the rifle or carbine.
These guys might be able to help. http://www.kragcollectorsassociation.org/