I buy most of my fish from Seaside in Huntington Beach. I've had great luck there. Other than them, I'll use Age of Aquariums in LB for some things.
I don't QT anymore. I do freshwater dip new additions to check for gill flukes. I have one of those holding tubs they use to collect your fish at the LFS. I'll hang that inside my sump, filled with RODI water with an air stone running. For a large fish/group of small fish or one that jumps, I'll cut the tub off a gallon of water and hold it in place, inside the sump with my mag float. This brings the RODI up to temp and gives enough time to aerate the water, reducing ph shock for some fish. While thats going on my new addition is temp acclimating in its bag in the sump. After 30-45 minutes of aeration and temp adjustment, I'll add the fish to the RODI. It sits for 5 minutes in there. I'll watch the whole time. Some will play dead. I just watch the gills, breathing will be stressed so I'm watching to make sure the breathing doesn't drop off and slow. Times up I check the container for any fallen flukes.
If no flukes they go into an acclimation box. That lasts a couple days up to a few weeks. My scopas and foxface took sometime for my powder brown to lose interest.
From there it's high quality foods as a staple. Rods Reef or LRS is what I feed for frozen. I have Cool Mysis and their veggie version mixed up in my auto feeder.
I got tired of QT. I had great luck with it for a while but for some reason started striking out big time. I felt bad killing so many fish so I just started doing what I stated above. I'll say, the FW dip is huge even if you're going into QT. If you do have flukes or velvet is can buy you some time giving temporary relief. That can let you get them eating; and raise levels (if using copper) slowly without issue.
Before, I always did full copper treatments with CopperPower. It works with the hannah copper kit well. Prazipro, metroplex/focus for internal parasites.