The Cheapo can of 134 from Walmart is your starting point. If you have lost a lot of 134, you have a leak somewhere (seals most likely). Leaks do more than just let the coolant out, they let moisture in. If you have moisture in your system, you will just ruin other parts (like your very expensive compressor) when you run your system after you charge it.
If your compressor is engaging before you put the Walmart coolant in, you might be okay, but if your compressor is not engaging now, you got issues.
Troubleshooting on your own is okay, but if you don't have the right tools (like an evacuation system and gauges) you are just P _ _ _ ing in the wind trying to fix the system on your own.
Some things are just better handled by professionals now a days...