I have drop ceiling that has 2x2 clear plastic tiles which I need to replace. I have gone to Lowe's and Home Depot but their tiles are not 2x2 but 22"x22" or 23 3/4 x23 3/4. These tiles fall out. Do you know who sells 2'x2' tiles that actually measure 2'x2'

Never seen one. In fact it should be impossible to put in true 24x24. You need a bit of clearance for the thickness of the grid. Most are 23-3/4" or even 23-1/2" square.