Just looking at what I googled for NordicTrack bottom bracket, it looks like these are way overpriced compared to a regular bicycle BB. But I don't know if what I found was what you need, I just looked in general. If it was mine, I'd pull the BB apart and take it to the local bike shop to see if they could identify it. I wouldn't think NordicTrack would have a custom made BB for their equipment but I could be wrong.
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"It is the unknown around the corner that turns my wheels." -- Heinz Stücke