Try your local scrap yard. I have made lots of gongs for cheap by going the scrap yard route. Sometimes they can tell which is the hardest stuff. If they cut the hardened stuff with a torch it will ruin the hardening for about an inch in from the edges.
You didn't specify for rifle or handgun. There is some overlap in sizes, but mostly you need different sizes for hand and long guns.
Shooting handguns at steel within 10 to 15 yards you WILL get splattered frequently. At longer distances you will occasionally catch some lead. This can be avoided to some extent by good design.
Especially for handguns, mount your bracket to hang the gong on the back at the top. This will cause the steel plate to tilt toward the shooter at a very slight angle, and the lead splash will generally go down.
Do you have access to a welder? If not I can show you how to make a good stand with just a saw and a screwdriver.
Does your gong and stand need to be portable?
You can email me if you want specifics.
Vic out.