Refrigerant can be bought in small bottles called recharging kits at Auto Part stores and online, such as this kit on Amazon.
There are different types of refrigerant used such a R-134a which is used on newer cars and R-12 which was used in a 80s and 90s.
If your car or truck was made after 1994 than it likely uses R134a.
Before 1994 R-12 was the main Refrigerant used in cars and trucks.
If your car uses R-12 than it can still be bought although harder to find. There are duplicate R-12 refrigerants that can be mixed with a existing R-12 system such as this kit.
< Most shops recommend upgrading a R-12 system to the new R-134a which is a costly upgrade. To find out the type of Refrigerant your car uses look under the hood for a label or simply Google your make and model for the Refrigerant type used. The Refrigerant cans that come with a recharging kit will have a gauge built into it that shows the pressure of the system.
The cans will have a easy-connect that goes to the low pressure Refrigerant connector of the car.
Take car when handling Refrigerant
Remember Refrigerant refrigerant is a contaminant and can be harmful.
Never smoke around Refrigerant as it can turn deadly when it burns and enters the lungs.
A bursts of Refrigerant will freeze anything close including body parts possibly causing frostbite.
Always wear goggles as a safety pre-caution.
If in doubt take it to a shop.