I stopper on a garden tub with a delta faucet. The stopper pushes down and locks to stop water. It takes a few tries pushing it down to get it to stop and hold water. What is the best way to fix it or can I just buy the stopper?

In my part of the country, the type of drain you refer to is commonly called a tip-toe drain because you can use your toes to open and close the drain. Their locking ability diminishes over time and use, and eventually won't lock at all. They cannot be repaired, and must be replaced. You must remove the one you have to get the correct replacement because there are variations in the thread diameter at the bottom of the stopper.
You will need to use pliers large enough to grab around the white plastic underneath the top of the stopper that appears to disappear into the drain. Once you have the white plastic shaft secured with pliers, grab the very top (the chrome part) and twist it counterclockwise to remove it. Once that is off you will see a round piece of brass that looks like the top of a very large screw. IT IS! Using a stantard screw driver, again turn counterclockwise to finish removing the rest of the stopper.
A good hardware store should have a replacement for you. Be sure to match up the thread diameter at the bottom of the stopper.
To reinstall, you can reverse the steps or simply thread the complete assembly into place.