1.18 and later, coal is exceedingly abundant at higher elevations. (Y 100+) But just going caving above Y 0 yields lots. There is no coal below Y 0. You definitely want to get a Fortune 3 pick ASAP.
Also, examine why you are having issues with coal. If you use too much of it for smelting, switch to lava buckets. With a bit of dripstone and iron, you can create infinite lava farm right next to your smelting setup. Do not spam torches as often. You can use charcoal (made from wood) for torches if needed.