I am driving a Hyundai Elantra 2017, 9 months and 7200 miles old. I drove the car over a curb approximately four months ago. In the past two weeks, I noticed a tire bulge on the right front tire (Kumho brand), approximately the size of a baseball. I have minimized the car use and understand that the tire needs to be replaced right away. I understand that the safest thing to do is to replace all the four tires, but is it okay to replace the front two or just the front right tire? Is it possible to buy a used tire and replace it?
Replace just the front two. I don't recommend used tires because it would be near impossible to find a tire that is in the same condition(age, tread, tread patter, same brand, etc)as your other front tire.
You only need to replace all four tires in vehicles with all wheel drive systems. These can be negatively affected by having different tires in front and back, but for your Elantra which is front wheel drive, you can replace them in pairs.