First of course is to identify where it's leaking. Might be just a bad stem, or a poorly seated bead; in which case a new tire isn't necessary. Could also be a leaking wheel (ditto). If it's a nail through the tread, the tire can be repaired cheap.
Side note,
America's Tire Co (aka "Discount Tire" in other parts of the country) got me as a repeat customer by repairing a puncture on someone else's tire for free. (NFI)
If you must replace just one tire, be sure to put it on the front. Differences in rolling radius can cause the differential to wear faster (and it's a lot more expensive than tires).
Personally, I'd buy 2 new tires and move the one good one to the spare. Don't like to mix tires on the same axle, but mixing front/rear is usually OK.
Try it out on some hard turns in a safe place; then if the handling is unpleasant, switch them front/rear. Small differences in sidewall compliance, etc. can make a noticeable difference in how the car performs near it's limits.
I shouldn't admit this; but I have been known to use "Fix a Flat" to extend the life of a leaky tire. Planned to replace the tire at my next paycheck, but wound up waiting over a year and it still held air. Don't try this at home !