Are Pressure Sensors available that can measure pressure difference?

Yes, E8M-A1 Pressure Sensors and E8Y series can measure pressure difference.

Example: Connect the larger absolute value to the positive (+) port. 

Using the E8M-A1

Difference between pressure values -55 Pa and -20 Pa

Negative (-) port: -20 Pa

Positive (+) port: -55 Pa

Difference between pressure values 10 Pa and 25 Pa

Negative (-) port: 10 Pa

Positive (+) port: 25 Pa

The reference port is the negative (-) port. Use the port as the negative (-) port when it is left open to atmospheric pressure.