What do you mean prepare them? If you mean drilling, I drill round rod all the time and it is no problem, nor is centering the rod. You might want to give yourself a little allowance for drilling.
Or buy 3/4" blanks and cut 1/4" off on the band saw. I do this with segmented pen blanks all the time and it (almost) always glues up square. A sharp 1/4" blade, clean rubber wheels, and a slow, steady feed-rate will leave a smooth surface on almost anything.
Thanks everyone. It's expected someone to sell them. 5/8 is typical but not 1/2.
The problem is that I have a custom made jig to drill for a specific reason. It will only fit 1/2 inch blanks. the guy who made the jig is long gone so I don't know how to get another one made.
I will definitely email Dawn and see if they can make them.
While I am still not sure what you are trying to do, it sounds like you should purchase ANY blank, then "square it" on a tablesaw, bringing it to your exact dimension as you square---or running it through a thickness sander to final dimension.
That way YOU control it's exact size to make it fit your jig.
I make some 5/8" blanks that are prenty close to square, and can do them in any color you would like. I'd be willing to cut blanks to whatever size you need.
Sorry, didn't see that you had found some. Dawn makes some very nice blanks.