Stair tread blanks are available at home centers. Because there are so many different dimensions that can be used for stairs on a straight run, let alone of circular stairs, I doubt that you will find anything that will be usable without a lot of custom cutting. In any case, I do not think that just putting another layer of wood over your current treads would be the way to go. This will cause problems with tread height on the first and last treads and could cause code issues as well as be a tripping hazard.
I replaced a set of treads about a year ago and it was pretty easy. Rather than using the stock treads, I bought 5/4 lumber planed to 1-1/8", cut it to length and routed a roundover on one edge. This saved about half the cost of the stock treads. Then I prefinished the treads, took out the old ones and installed the new ones. I had very little sawing to do on the jobsite and the only finishing was to fill the nail holes. The pine that was there lasted 150 years and the new oak ones should last much longer. You might want to consider something like this.