I have TONS of clip back findings......flat ones, ball top, etc.
Give me your address (do not post it here, just reply to me) and I'll send you
some.

What I have found:
Those converter 'kits' or findings don't work. When bending the posts, they
tend to not lay flat.....they are a distance from the ear lobe.
Depends on the earring.

If it is post....I just clip off the post and E6000 to a clip finding. I have
done this with 14K gold and, after curing, the glue is VERY durable and long
lasting. I have some that are more than 6 years old.
Sometimes, I can just 'hook' it onto a finding .....if the finding has a 'loop'
to hook.
Everything depends on the earring itself.

If I'm making them, I adapt the design to the finding.