I bought a packet of 100 on ebay for like, a dollar. My local store wanted the exact same ones for $2.50 each!
I do break them a fair bit, but the trick is to hold them firmly where the wire connects, so that you're pressing it hard against the backing plate. This makes them much harder to pull apart accidentally. A drop of PVA glue would probably help too, but I find the metal is too smooth to really attach properly, and the friction of your thumb helps. Strong small magnets/clip with some rubber in between might work well too?