I'd like to know where they go too...I like to have lots of bobbins but seems like most of the time when I get a machine, even if I get a box of accessories there's only 1 or 2 bobbins in there if I'm lucky. Usually there's just the one that's in the machine.
I did find a good supplier for NEW metal class 66's that actually fit and work in my various old Singers; they're really good so I bought 30 of them before they could vanish on me! I need Pfaff 15's too, I only have one. One bobbin is not enough!