Any decent leather shop should be able to make one.
There are a number of leather workers on Etsy that advertise them.
https://www.etsy.com/listing/576974475/handmade-leather-cartridge-holder-rifle?gpla=1&gao=1&&utm_source=bing&utm_medium=cpc&utm_campaign=shopping_us_-accessories-domestic_low&utm_custom1=_k_39a7fef1379c1fb49af09fe69999010c_k_&utm_content=bing_412372785_1299623041851236_81226503829015_pla-4584826055760828:pla-4584826055760828_c__576974475&utm_custom2=412372785&msclkid=39a7fef1379c1fb49af09fe69999010c
