The decent ones have a metal spacer between the 2 nuts and a couple of small straps holding them together like the ones below.
You'll have to have a good shop around for a good seller.
If you have the spacer missing then they are not going to be much cop because the second nut moves freely.
Get the spacered ones, they are much better. And you can tighten them up a bit with thin aluminium foil between the spacer and the slave nut.
You can try to diy the ones you have but it'll be a bit tricky.
With spacer
Without spacer: (junk)