Most online stores will have single drip tips, in all kinds of material, color and design. They range quite a bit in price, depending. Some are quite cheap. Shipping may be more than the cost of a single drip tip...so maybe remember to add some to another order.
Or, if you have a local
vape shop, most of them sell them as well. While they'd be a little more there, no shipping fee.
You can also try replacing the O-ring on the tip you have. Again, online or a vape
shop. Or maybe the plumbing area of a hardware store.
Some have also carefully taken off the O-ring, wrapped a turn or two of thread in the groove and then replaced the O-ring for a tighter fit.