I’ve had good experiences buying plants from black jungle, neherp, and California carnivores. For larger stuff, I got a morninga tree that did well from somewhere online. Can’t remember where maybe toptropicals(have heard mixed reviews, but I had no complaints personally).
Locally from Home Depot, Lowe’s, Costco, etc is probably the cheapest bet, but they usually* don’t have anything that rare. IME they do carry pretty nice stuff though. I love some of our nurseries and they can have unique stuff as well, but it’s usually at top dollar. Another good option is Facebook/local trades and such.