herkygrrl and monkeydaddy are right. Typically its the nails that rust away, causing the (mostly) intact slate tile to no longer be attached, and fall off. You probably have more than a few tiles that are tenuously attached and close to falling off. This means that unless the tiles that need to be replaced are at the very edge of the roof, when the roofer goes up on your roof to repair it, his choices are to either set up staging that keeps him from actually touching the roof (costly, because it takes a while to set up, and he could charge you to “rent” the equipment, although its basically like playing with legos, so its not an advanced skill), or you accept the fact that he will knock more tiles off while up there, and replacing those additional tiles will also drive the cost up. With that in mind, I would wait until you have to replace enough tiles to justify the cost. And if you’re planning on renovating in the near future anyway, waiting until then is the choice I would personnaly make.
If you do choose to repair the roof, I would only consider “custom cut” tiles if you will be able to see the difference in shape from ground level. The difference is purely aesthetics, so you may not even see the difference.