How can I find replacement part for Mongoose scooter?
It looks like a quick release seatpost binder

You might have a bicycle with one on it you could use to test the fit.

Googling "bicycle seatpost quick release M6 × 65mm" gets results
Here's one on ebay as an example (not a recommendation)

Or, you might be able to find a metric bolt with the correct thread down at the hardware store along with appropriate washers. Tighten it with a wrench rather than have a quick release lever as a temporary fix.