I got all of my once fired hulls by buying boxes of shells and eventually shooting them all.
I'm sure most of us started exactly where you find yourself right now.
There's a bunch of commercial loads available right now though.
Be grateful you didn't pick up the 16 gauge back in the early 00's when you couldn't hardly find a box if your life depended on it. Though, I remember in the late 80's and early 90's, you could walk into any Kmart or box store and get a box of high brass for $5.97
Naturally, the prices now are 3-6x higher than back then. Cheapest I'm seeing right now is ~$15/box and it goes up from there
But if you're looking to buy:
https://www.ammobuy.com/ammo/16-gauge
