Are capture nuts and cage nuts one and the same? Most cities have a specialty store that carries such. I get everything from a shop in Lafayette, LA, A-1 Bolt, that has every conceivable fastener know to man. I just bought a large handful of grade-8 fasteners for less than 5 bucks.
On the subject of fasteners, my new gear reduction starter has two long, thin recessed fasteners that hold the adapter plate onto the body of the starter. I brought these to A-1 to see if I could find some 5mm longer so that I could use a nut backup. These bolts the counter-guy told me were indeed grade 12.5, the strongest made. I find it curious that such a strong bolt would have fine threads and be threading into aluminum yet. Hmmmm. I guess it is the shear strength that they want, and not the elongation, as the aluminum would certainly give way before the bolt. Was advised to us high-strength Locktite to keep them from backing out. That is why I wanted backup nuts. (they were out of stock)