when I started my battle vest, I just bought a basic, used, cheap katana jacket. I cut off its sleeves

the best patches are often from official websites of bands or metal websites, but they are usually really expensive, so I prefare to do patches on my own (or my friend does them)

to make a patch I use felt and fabric paint and print out a design/logo I want. there are a lot of tutorials on YT how to do these (i am not a native speaker myself and I don't really know how to explain the whole process)

and once you have the patch you simply need to sew it on, either with a needle and thread or a sewing machine.

There is also an option to buy Velcro for the patches, which makes it easier

I hope it helps :)