I get asked about this a lot so I'm creating a blog entry dedicated to it. I currently do not know of any gold sequin fabric sources. I get lucky online once in a blue moon, or a friend may find something random, but I do not know of any place that stocks it.
Rubies makes a gold sequin tail coat for men (item 90872) and women (90871) that retails for about $250 - $300. They turn up on eBay fairly regularly as well as large costume companies.
Every once in a while you can catch a good vintage find that comes close. I guess it depends how picky you are about the sequin design and jacket shape.
Most common these days are the 'foil dot' costumes - faux sequins glued onto a glitzy fabric background. A reasonable investment for a starter costume or someone who doesn't plan to perform Columbia often.
If you're dedicated to accuracy you'll want to look for a wavy, skip-a-row sequin arrangement. I've seen the wavy strands attached to chiffon, taffeta, jersey knit and thick spring weave (think tube tops). I am not a fan of using stretchy fabrics for tail coats and the only advice I will ever give on the subject no matter how hard you press me is "no" - not because it can't be done, but because I don't want to be responsible for a multi-hundred dollar failure if you don't achieve the results you wanted.
I used to find the taffeta-backed version at Hancock's regularly until about 10 years ago. It was discontinued and doesn't seem to be coming back. I find a few bolts in random fabric shops online, but never any options of restocking. Magic Makers used to carry it regularly, but as prices went up he stopped having it manufactured.
If you're less concerned with the accuracy, there's still plenty of gold options out there. Sometimes with square or oval sequin, sometimes dangly instead of stitched flat, etc. But for the cost of buying something that isn't right and putting that much work into it, you'd probably be better off buying the Rubies coat mentioned above.
I'm not currently accepting custom orders - partially because I can't find a reliable fabric source myself, and also because I've suspended all commissions while I'm caring for my mother and trying to move forward with other (non-Rocky) work.
However, there has been some dialogue about having it custom manufactured like the bustier fabric is. The problem, of course, is the huge amount of money that has to fronted by an individual for a bolt of custom fabric. We're talking a couple thousand dollars. Not a big problem if it gets unloaded quickly, but definitely a concern if it takes months or years to recover the cost. It would most likely be in the neighborhood of $60/yd, and take 3 yards to make a tail coat (not including black sequin fabric for the lapels and whatever fabric for the lining).
If you think you would seriously consider purchasing custom fabric in that price range I encourage you to leave a comment to this blog entry. If enough people are seriously interested we could make this a reality. Please, think of the Columbias.