Your best experience will be gained by working at a garden center although workers are rarely privy to the business side of running that kind of concern which is so dependent on weather,costs of purchasing stock in small quantities,finding annual financing at a reasonable rate and finding laborers who are reliable and can speak English.

We all are tempted to buy from the mass displays at the big box stores and the lower prices let us buy even more there.
This is key to killing off the smaller,more knowledgeable garden centers which are ,as you said, few and far between.
They can't spare the $$ for advertising so rely heavily on word of mouth.
Some (no, many) smaller garden centers try to be all things to everyone and spend too much of their budget on lawn-care items,fertilizers and insecticides as well as offering commonplace plant materials.
Carrying forward inventory year to year it degrades and winds up as a loss. For some reason, I never see the small centers offering deep discounts on these kinds of items so they can buy fresh for the following year.

and..first think about why you were disappointed when you visited the garden centers in your area.
What couldn't you find..specific color or variety or just the limited offering of species?
Define what qualities were missing. Expertise? Using staff who were not plant experts or seemed uninterested in your questions?
Expertise costs $$ and often you get someone with book learning but no experience in gardening in this area and zone.
I've yet to find one source for all my yard and plant needs but the search is sometimes fun although time consuming.