Slime. Use a little more than recommended on the bottle. I have tried, but I still don't understand the advantage of tubeless tires under more or less normal conditions. Best I can tell, the advantage of tubeless is that they, by default, use sealant, and they are often easier to repair when they have flats, which they do. The latter is no small point, especially if you're racing, but adding sealant to the tires you have will make them as flat resistant as tubeless tires.
Maybe, when your tires need to be replaced anyway, consider going tubeless. Forego the expense otherwise and just add Slime.
TT