If you want to be fancy you can call it a "Call Sight" - there were some factory guns with this as an option, according to SCSW (it's in the glossary).

Depending on how much you want to spend, you could get a piece of brass rod from a local hardware store ($2-3) or a see a local jeweler about a small piece of 14-18k gold ($??). I'm sure the jeweler would be able to make it in the size and shape you wanted without any trouble.