First thing that comes to mind would be to buy cheap molding from Lowes, miter the corners to create a frame to the size needed then embellish with other pieces of cheap trim and decorative scrollwork from craft stores. Paint it black, then dry brush with gold paint. Or just check out thrift stores and flea markets for frames.
New moulding can be expensive. I'd go the Goodwill/Salvation Army route. Also, check out those construction salvage/recycler places. They often have odd lots of mouldings of various designs and lengths at a fraction of the cost of those found at the big box retailers. Don't forget, you can also fake a large ornate moulding by adding thinner fancy stuff onto a plain board.
Like the rest said, thrift stores, garage sales really anyplace were they are selling or getting rid of old junk. Try asking around in your community, i have made a lot of good contacts that way and have gotten a lot of cool, hard to find items, like for example last year a local dentist gave me a bag full of plaster teeth moldings and a dentist drill for free. Try it, you never know.
you can also look for a molding warehouse that sells framing molding and supplies in your state. ive cut down and repurposed frames or made my own with regular trim molding, usually you can find victorian trim in lowes, home depot discontinued carrying that..............any plaster ornate frame with a ding in it, repair it with plaster, just free form it or you can make a silicon mold and repour it and glue it in..distressing anything also is easy, buy the gold or silver flake paint at michaels and layer your base color and rub on, stain etc...its fun!!