A Internet search reveals:

A Search in a Search:

https://www.bestdeals.today/sig-p320-magazine-base-plate?origin=google&google_params[matchtype]=b&google_params[network]=s&google_params[device]=c&google_params[creative]=304183092860&google_params[keyword]=sig%20p320%20magazine%20base%20plate&google_params[adposition]=2t1&google_params[adgroupid]=59729568919&google_params[campaignid]=1601187085&bs=GZIQqjlzd_fBO6RZQS0fbAK3aR7JQTpBHFyjaWQ0D0CznYnrBWKwTPiaw2UtlVxaJNGr-ZUj4C4oOLbQbWFK0hAI2AE8Uu9V&google_params[feeditemid]=&google_params[targetid]=kwd-375618764536&google_params[loc_interest_ms]=&google_params[loc_physical_ms]=9011974&google_params[devicemodel]=&google_params[target]=&category=13&dest=0&sys_id=0|584&gclid=EAIaIQobChMI3cK33-eQ3wIVmYqzCh2SrggsEAAYASAAEgJ_1_D_BwE

NDZ Performance:

https://www.ndzperformance.com/Sig-Sauer-P320-Accessories-s/18389.htm

Springer Research:

https://shop.springerprecision.com/P320-Basepads_c85.htm

ANR Design:

https://www.anrdesignkydexholster.com/product/sig-sauer-p320-carry-magazine-baseplate/

Taran Tactical:

https://tarantacticalinnovations.com/magazine-extensions/sig-p320/

Obsidian Arms:

https://obsidianarms.com/product-category/magazine-base-pads/http://

Grayguns:

https://grayguns.com/product/p320-25-basepad/

Henning Group:

https://www.henningshop.com/Detail.aspx?PROD=1291104&CAT=10068

Mantis:
Magazine floorplate with rail

https://mantisx.com/products/magrail-sig-sauer-p320-p250-magazine-floor-plate-rail-adapter

That should get you started..Happy Hunting!

 

What are you looking for in an aftermarket base plate? Are you going for aesthetics or functionality? I see them all over the place for lots of guns, but I've not used them before.