Never heard of anyone rebuilding ignition switches and a quick search only turned up links on how to replace the whole switch. Rebuilding was never cost effective as replacements were readily available.
E-Bay--
https://www.ebay.com/itm/New-US-Made-Original-Ignition-Switch-GM-1116676-Pontiac-GTO-LeMans-Tempest-66-67-/302645477046 . This stuff is pricey but that’s the market today.
You might try
corvettestop.com. Yes it’s a vette place and they restore/rebuild vette parts, but some items are common to other GM cars, like alternators, wiper and blower motors, etc. which they rebuild. They don’t mention ignition switches but their electrical guru may be able to help—worth a shot.