Any wideband O2 with a 0-5v output can be used.

The Innovate LC1 and AEM Uego are great examples of WBO2 sensors that work great with the Flash Ecus.

After following installation instructions for your choice sensor you will need to wire the 0-5V output (see your WBO2 sensor manual for wire color) into the ECU. 

Wire the 0-5V into pin 93 for a 98-99 Flash Ecu, pin 55 for a 91-93 Flash Ecu or pin 77 for a 94-97 Flash Ecu.

Then just finish your sensor scaling in the rom and you are set up. http://chromedecu.org/?page_id=895

See plug pinouts below. The diagrams show if you are looking directly at the ecu plug, or the back vehicle side plugs.