If I'm interpreting it correctly, a spring belt is essentially a very
long coil spring that forms a belt.  These were used in home movie
projectors and I think they're still available for that purpose.

Two possible sources are Russell Industries (the old Projector-Recorder
Belt Company) and there's this place that sells light bulbs for old
movie projectors -- lightbulbs.com or something like that.

Belt lengths are typically not specified because these are sold as
replacement parts for certain brands of projectors.  However, the firms
are generally helpful, and if you need a longer belt it's easy enough
to splice in a length of spring from another belt.

Mark Kinsler