You don't need a special tool to measure your sit bones. Here's how at home:

1. Put the toilet seat down.
2. Spread a hand towel over it.
3. Put a sheet of aluminum foil over the towel, try not to wrinkle it much.
4. Wearing your tighty whities, sit down in a position similar to how you'd be on the bike (your back should be leaning forward similarly), and don't move around.
5. Stand up and observe 2 dimples in the foil. Mark their centers with a sharpie pen and measure.