I've been making my own soundpost blanks with a
tool like this one for at least 30 years. I just set the fence on my bandsaw to about 1/16" larger than the finished diameter and saw square spruce stock about 3"-4" longer than the desired post length. When I have finished sawing the stock, I put the drive in my lathe or drill press chuck. While the drive is turning, it's just a matter of placing one end of the stock into the turning drive and push the hand held cutter into the turning wood stock. (It's easier to do than to describe!). I can turnout about post blank in less than a minute. All that is required to finish the post is to cut off the 3-4" end that was in the drive and a quick sanding. I can make enough posts for several years in less than an hour from start to finish.
The tool has replaceable cutter inserts in sizes from about 1/4" to over 1" in 1 mm increments. I usually use a 19 mm for basses, but I have larger and smaller and larger inserts if I need other sizes. This is a well made, heavy duty tool.
The bad news is that the tool is rather expensive and to the best of my knowledge, it is not available in the USA. However, if you need to make a lot of soundpost blanks, I doubt if there is an easier or faster way to do it.