Yes you can buy a calliper on its own, but you might be better starting off by giving them a bleed and fitting a 180 rotor (you’ll need a five quid adapter and a new rotor).
If that still doesn’t word I’d say just buy a new front brake set (lever, hose, calliper). SLX are good.
*ninja edit, if you do decide to just buy a calliper, it has to be another shimano one to work with your existing lever.