Unfortunately most of the pre-made spey handles are crap. I used one on the first rod I ever built, and the crappy cork kind of went with the $30 blank, so no problem.
But you can’t turn them to shape them further as they can most charitably described as “cork pieces held together with wood filler”. They will fall to pieces if you try. They look nice in the pictures on mudhole, or wherever, but they are very crappy and even without reshaping they will eventually fall apart if used heavily.
Grips are pretty easy to make and turn yourself, but the overhead cost and effort of setting up an actual or Rube Goldberg style lathe may seem like way too much if you just want to do one or two. So it comes back to having your local rod maker do them on the blank of your choice.
Depends on where you live, but if you are far enough to have to communicate via email and fedex you can use just about anyone. I always use Steve Godshall who is in the Medford, OR area (about 6 hours drive from me) when I want something really special, or something I tried to do myself but didn’t get exactly right. He does beautiful work, and the only limitations are your own imagination. If you know the exact shape and dimensions you want you can just send a drawing. But I’d recommend you get it glued and turned on the blank itself (with the reel seat) as reaming out a stand-alone, spey length grip can be a huge pain. You can still do all the rest yourself.
FWIW I have both let Steve pick out the cork he felt best fit my requirements himself for some projects, and have provided all the cork myself on another. On a couple of them I sent him the handle of another rod and said “this is the exact shape I want”. It’s all up to your needs at to how much or little you want to tell whoever puts it together for you.