As Tony said look at the cios website and if necessary contact them ,they were very helpful when I did.Richard Heaumes museum in Forest is very good as is the La Vallete military museum in St Peterport,fort Hommet is also well worth a look and also battery Dolmen with its fire control tower.
there are also many bunkers etc spread around the island many of which are easily accesible although they have been stripped of most fittings they are quite clean inside and have suffered little or no vandalism so are worth seeing if such things interest you.
When in St Peterport visit thr Guernsey press shop as they have a good selection of books on the occupation which will make finding things easier.
As for scrap yards I cant remember any and to be honest on such a small island with so many collectors I would not get your hopes up for finding any hidden gems
Nigel