I checked with Greenzone since they are still in business. Although they don't carry the tandems anymore they still have a few sets of the IPS cranks.
They work with standard square taper bottom bracket. They offered to sell me 2 IPS cranks (right side - front and back) for $99. This price does not include left side crank arms.
To get the complete set including matching left side crank arms it would be $149 shipped. You can reach out to: alan@greenzonebikes.com.