I get empty gelcaps from amazon and local nutrition food stores. They cost like $5 for between 100-1000. You can get them in a wide variety of sizes, though you might have trouble if you're looking for something larger than size "000" (1cm diameter, 2.6cm length). Here is a size chart[0] (but there are cheaper sources for the gelcaps).

0: https://ibspot.com/products/capsuline-size-1-empty-capsules-...