Are you talking about the clips that look like normal quick links only they stay shut with a spring?
I can only find them online already attached to toys--maybe you could buy some links from them or find out where they get them.
If you can't find that, there is a type of snap called a bull snap that is frequently used for horse lead ropes. The inner section folds out and is much harder to open than a dog leash snap. They are very cheap (under $2 each) thru horse tack shops or cataloges.
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