First to 410er, congrats. to good ole boy. There isn't much reasonable on the market. New Italian doubles aren't cheap. Used can be had. A custom flint smoothbore will do well. Such as a smoothbore trade gun. If you find an older rifle with a rusted up barrel, a gun smith can ream the bore up a few thous to a smooth bore. Or get a TC or Lyman and a drop in smooth barrel which are about like finding a running Yugo for sale.
There are some snazzie looking smoothies being imported from India. (Yeah I know!) Check Middlesex Village Trading Company's web site.
At one time there were some resonable single barrel percussion guns made in Spain. They come up used on the on-line auctions every few months.
I saw a used nAVY Arms double percc at Cabela's in HAMBURG PA IN FAIR SHAPE ABOUT A YEAR AGO FOR $500.
Finding an oldie and fixing it is an option, but requires careful selection and usually requires new internal lock parts (The half cocks are usually wore out and don't work) If you do look at oldies to fix, be wary. Many in private hand are still loaded. I have fixed an oldie 12 double. It is a joy to use. You need enough barrel wall. Some cheaper oldies were like stove pipe.
Once you shoot an old origional one,a new one just doesn't feel the same.Yes it is very hard to find one in shootable shape.I have been blessed this year with finding both a 11ga and a 20ga.The search continues!