Looking on Hatsan's website they don't list what magazines are compatible, they have a product page that shows pictures
https://escortshotgunsusa.com/product/box-magazines/
But there is no item number, or link to buy, or even any info on compatible other brands.
I found a couple on atlanticfirearms.com that look similar, but I can't t say for sure they'd fit or function
https://atlanticfirearms.com/products/ifc-magazine-13-round-410-2-5-for-ifc-410-upper-receivers-410mag13
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Id contact HatsanUSA, Inc.
P.O. Box 576
Bentonville, AR 72712
Phone: 1.877.278.4448
Or buy using the "contact us" form on their website
https://escortshotgunsusa.com/contact-us/

Matthew, there are no current after market magazines. Hopefully they will be coming soon as this will be a popular model and 3rd parties will likely jump on the band wagon. Cheers and stay safe!