There will be local laws dictating what you can and what you cant put in the A/C system. And as Wheeler says, you might also need to be certified to do the job.
Here in Australia, you can not buy R134a or R1234YF without being certified. Refrigerant is only available wholesale, it can not be bought retail. So you need to be a business AND certified to buy refrigerant.
"Topping up" the refrigerant is not a good idea. The A/C system needs a particular amount of refrigerant to operate efficiently. Too much or too little and the system wont run as well as it should and may even cause problems.
The amount is determined by weight, either pounds or grams. If you only have a partial charge in the system how do you determine how much to add?
The professionals have machines that weigh the amount of refrigerant left in the system when the system is drained. Then they refill the system with the correct amount. Unfortunately you don't usually get a credit for the refrigerant removed from the system! So the cost is usually the same, whether there was gas left in the system or not. Professionals will never top up the system.
Some businesses reuse the refrigerant drained from A/C systems. Some will decant the refrigerant into a "spare" container and send it off to be recycled. There is money to be made in recycling refrigerant!
R134a and R1234YF have different service port fittings and they are incompatible.
In some places a Propane/Butane refrigerant mix is legal. In some places it's not. I will not be drawn into any arguments for or against Propane/Butane refrigerant!
Different types of refrigerant should not be mixed. Usually it MIGHT be possible to to run a system on different refrigerants, but, they must never be mixed. Which is another reason why the A/C system should be completely drained before adding refrigerant. The actual refrigerant in the system and what you think is in the system might not be the same thing!
Some cars suffer defects in the condenser (the radiator like part in the front of the car) where heat cycling cracks joints or welds. The crimped joints between flexible and rigid pipes can leak. The "usual suspects" for leaks in the A/C system are the schrader valves in the service ports. They should be replaced when the system is serviced. The caps on the service ports are NOT leak proof! If you have a leaking schrader valve, tightening the cap wont help! If you remove the caps on the service ports and you get a quick "ssst", the valves are leaking.