In the past, we've ordered custom SOI wafers 25 at a time from Silicon Valley Microelectronics (https://svmi.com/) with the following specifications:

Device Layer:
    Diameter: 150mm ±0.5mm
    Thickness: 40um ±1um
    Type/Dopant: P/Boron
    Device Resistivity: 15-25 ohm-cm
    Crystal Orientation: <100> ±0.5 degree
    Front Side Finish: Polished
    Primary Flat Orientation: <110> ± 1 degree
BOX:
    Thickness: 2um ±0.1um
Handle Layer:
    Resistivity: 15-25 ohm-cm
    Thickness: 550 ± 15um
    Back Surface: Etched (or Polished) with oxide
    Crystal Orientation: <100> ±5 degree or better
    Primary Flat Orientation: <110> ± 5 degree or better
    Back side finish: no oxide