Pretty sure that should you be unable to source R22 for your residence you’re in for a new coil that can handle R410a at the very least.
Suspect I’m going to wish I’d secured a solid option on R22 for my residence since I opted for a new unit the last year they could sell them and didn’t want to cough up for the new inside coil. Looks like it’s going to be available until 2020, but who knows if AC service firms will be able to use it after then.
You can still get R22, you won’t like the price at wholesale of about 23$ a lb. Even that price is constantly changing. You can also still get R12 if you want it. I heard of the MO99 being a drop in or even top off. But I can’t vouch for it as I don’t work with that much. The last R22 machine that I was involved in at work putting in was in about 2004. It’s mostly been 95% 134a.
I found a virgin 30lb jug of R22 for $600…ouch!
I’ve got an R22 unit that was put in about 10 years ago. The service valve leaks slightly. I remember the tech brazing it up without a heat sink on that valve or wet sock and I guess it finally gave out. I put a gasket in and some leak lock on the cap, to see if it won’t leak. So far the sniffer says I’m good.
I actually switched out the large unit 4 years ago for an R410a compressor and left the old coil in as well. So far, it has held its charge this long with no issues.
I went ahead and got my EPA 608 certs so I can do it myself.
Ideally yes, but I believe the state still mandates that you have to have a License or work for someone with a license. Many supply houses won’t sell to you with just a cert.
Your right about that wholesale supply house like Johnstone not selling to you. But good thing, there are other choices. You can’t sell your services, but you can work on your own equipment. They haven’t outlawed you yet to work on your own home !! 
Recently did a black market deal for three cans of R12 for some used VW auto parts. The good stuff holds it’s value!
I gather that as the moratorium on new R22 equipment loomed, the industry started selling coils that would work with either R22 or R410A. My house was built in 1992 and the coils are most certainly original, so I’m probably in for major equipment replacement if R22 isn’t available when I next need my system serviced.
At one point in the recent past I had a can of R12. Lost it in a recent-ish move, much to my chagrin. No idea how much it would be worth now in a cash-no-cops transaction.
My “source” said they were about $45. I paid a $1 a can. Wished my other investments had done as well.