The easy option is to take the tire your local tire shop. Most tire shops have a number of ways to seal tires that stubbornly refuse to fully seal.
First check the valve stem to see if it is tight and the leak is coming from there. You can put a film of soaky water on the valve and if it blows a bubble then tighten the valve stem with a tool to tightenen. You can get the tool from Tractor Supply or an auto parts store.
If it is not the valve stem then spray the tire, with soapy water, where the tire and rim seals and look for bubbles. If you see bubbles then inflate the tire to the maximum air pressure that is printed on the tire. Some tires will have a bead seating air pressure amount printed on the tire. This is the amount of air pressure to seat the bead ONLY and not operating pressure. Check for leaks with the soapy water. If this stops the leak allow the tire to sit over inflated for a 30 minutes to 2 hours. Then reduce the amount of air to the recommended operating pressure. Check to see if it is fully sealed.
In order for a bead to be considered seated the tire must be inflated all of the way out to the inside edge of the rim. If this has not happened you must lubricate and inflate to max air pressure to and allow to sit until this occurs.
If the tire still leaks deflate the tire fully and break the bead so that you can inspect and clean the bead. You may need to remove the valve stem in order to fully deflate. Compress the tire so that you can clean and inspect the rim and tire lip. Look for any dirt or debris between the tire and rim. Look for any defect in the tire or the rim. If the tire and rim are free of imperfections then it is not a tire or rim problem. Make sure that the tire and rim are clean. Rotate the tire on the rim so that the tire and rim is sealing at a different location than when it was deflated.
Inflate with air. Use lubricate when inflating. It will help the bead seal. If you cannot get the tire to inflate wrap a ratchet strap around the tire and tighten, forcing the tire lip closer to the rim bead sealing area and apply air.
CAUTION: DO NOT APPLY MUCH AIR. AS SOON AS THE TIRE STARTS TO SEAL RELEASE THE LOCK ON THE RATCHET STRAP. BE CAREFUL. IF YOU HAVE ALREADY INSTALLED TO MUCH AIR WHEN YOU RELEASE THE RATCHET STRAP YOU COULD BE INJURED. REMEMBER AS SOON AS AIR IS BEING RETAINED INSIDE OF THE TIRE IT IS APPLYING PRESSURE TO THE RATCHET STRAP MECHANISM AND WHEN IT IS RELEASES, IT WILL RELEASE WITH AN EQUAL AMOUNT OF FORCE TO THE AIR PRESSURE OF THE TIRE. BE CAREFUL WE DO NOT RECOMMEND YOU DOING THIS UNLESS YOU HAVE EXPERIENCE AND ARE WILLING TO TAKE THE RISK.
Once the tire is holding air the next challenge is to get the tire bead to seal against the rim. Do not over fill the tire in order to get the bead to seal. Each tire will have a MAXIMUM air pressure that can use used to get a seal printed on the tire. DO NOT EXCEED THIS LIMIT for any reason. Add lubricate to help the tire seal if necessary. CAUTION: KEEP YOUR FINGERS OUT OF THE BEAD SEALING AREA. WHEN THE TIRE SEALS IF YOUR FINGER GETS CAUGHT BETWEEN THE RIM AND TIRE YOU COULD RECEIVE A SEVER INJURY AND IF YOU TRY TO JERK YOUR FINGER OUT IT COULD CAUSE EVEN MORE INJURY.
If you follow everything above and there are no imperfections in the tire or rim and the tire still does not seal you can take to a tire shop and they will get it to seal. Or you can purchase inner tubes from places like Tractor Supply or eBay.