Hi,

This fell off the circuit board for my multi-tester yesterday. Can anyone identify what component it is and where I might get a replacement?

 

Welcome to AAC!

Looks like a crystal. They typically have the frequency marked on the can, but your picture doesn't have sufficient focus/resolution to see it.

 

It is probably the cover of the microprocessor crystal. The tester may still work but the leads to the crystal are very fragile and it will not remain un-damaged for long without its cover. If it still works, try to locate the crystal on the circuit board. It will be the same shape as the bit that fell off. If you are very careful, you can replace the cover and then use some kind of glue to hold it in place. Don't get glue inside the crystal housing.