The blue ultrasonics are not available anymore, ( unless somebody is holding on to a few)...
PDQ offers an upgrade kit, that repaces the blue ones with smaller black ones, and an extra board to communicate with the VT board. There is no longer a need to buy a D/S or P/S specific ultrasonic with the new system. The whole upgrade system is cheaper than buying 2 of the blue ultrasonics,(if you can find them)