Spring loaded ceiling grid.
I don't quite understand why you'd need or want such a system unless it was for outdoors (canopy on a commercial bldg. for example) where the wind might come into play and blow tiles out. I have used plastic clips in such a situation that slip over the grid and hold the tile. Inside, it shouldn't be necessary. Try a drywall/acoustical supply if you're sure you need some sort of "hold down" clips....
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