Phil,
I do not know where the bridges came from. Sometimes the D&RGW AFE (Authority for Expenditure) had that info and sometimes not. The Colorado RR Museum has the full AFEs from about 1939 on, but before that all they have are summary index cards which only give the cost, start and complete dates, and not much else.
I have some info on the bridges on the Silverton and from Chama to Alamosa that I found in a bridge book. The bridges over the Navajo near Gato came from the North Fork branch to Somerset. The truss bridge over the Animas at Durango came from north of Antonito and that bridge at Antonito came from the main line near Canon City. The girder part of the bridge at Durango came off the Pleasant Valley branch in Utah. As the D&RGW rebuilt the standard gauge main lines in the 20s for heavier motive power, they saved the old bridges for use on the narrow gauge or standard gauge branch lines. Almost every bridge on the narrow gauge is second hand.
I do not know where they stored the old bridges, D&RGW documentation does not provide that info.