I recently bought a couple of 10" toroids from HV Research and found
they are well made and a value for the money. They have a seam made by
rolling in the edge of the two halves but this does not affect coils
using a breakout such as a DRSSTC or VTTC (in my case).
I also noted that John Gudenas, the owner, mentioned custom machine work
available for TC or other HV items. I asked him to make a phenolic disc
for a series rotary I am making. His work is good, expeditious and very
reasonable price wise.
For those who lack lathes and milling machines you might consider
HVResearch.com.
(p.s. another list member John Freau also makes toroids in other sizes
and lists them on e-Bay from time to time. I have several and they are
very well made.)