Start simple, buy a standard Vernet tip (one size fits all. . . usually)
If you have exceptionally small, slender fingers & thumb you might need to inquire about a better fitting device. . . I think Vernet offers smaller and larger sizes now. If not, try a "Sixth Finger" gimmick. You can do most everything with that, that can be done with a TT and chances are strong that you'll fool magic bums with it because it's such a rarely used device.
There are people that make custom fit TTs as well as Rubber, Plastic & Metal ones but it is rare you'll ever need anything other than the Vernet. I've been doing this stuff since the early 1960s and use the Vernet gimmick far more than the more expensive rubber one's I've been given in that they get exceptionally dirty really fast and over time, get darker, no longer matching my Lilly white Dutch-Irish flesh. . . there's times when the contrast is just too much (though, when properly worked, you really can wear a bright red TT and no one will notice; I've seen it done and it's mind blowing.)
Hope that answers your question