i think now the only way to find parts is to buy a used gun from someone to fix your own.
I would go to Gun parts Corp. and find your JC Higgins model and exploaded view of your firearm.Then find the part number you need.they will tell you if they have it in stock.If not you may get the info you need to find them out in the general public.
At a good gunsmith's shop. Maybe Numrich's Gun Parts (search gunpartscorp) For the complete trigger guard assembly, including trigger, safety, springs, pins etc., you can get it from Numrich, Midway USA, Brownells or direct from Marlin. If it's just the trigger or other individual trigger guard parts you need, you'll have to get those from Numrich for a different model Marlin.
Google.
To find a trigger guard for a JC Higgins model 103.13, you can check online marketplaces like eBay or GunBroker, where used or vintage firearm parts are often sold. Additionally, consider reaching out to specialty gun shops or gunsmiths that focus on vintage firearms, as they may have parts or be able to order them for you. Finally, firearm forums and communities dedicated to vintage guns may provide leads or recommendations for sourcing specific parts.
Try gun shops, gun shows, egunparts, garage sales, estate sales, etc..
Try e-gunparts.com
Gander mountain usually has them.
www.histandard.info
Try e-gunparts.com
I have quite a few. sales@countrygunsmith.net
Marlin Firearms
You can find it on the Crosman web page. The JC Higgins model 180 was made by Crosman Air guns. See the link below