I have no idea about large rhododendrons at CA Home Depots, but since you live in an area of the country still blessed with many rhododendron nurseries, why not patronize them rather than a big box store where the plants have often received indifferent care? Home Depot may be cheaper, but the big box competition is one of the main reasons why independent growers and nurseries are in serious decline. If you're going to purchase large common rhododendrons at HD or another big box, it won't be long before the more interesting varieties currently produced by specialist nurseries will be impossible to find. The choice of where you buy rhododendrons is purely a personal decision, but big boxes are a matter of valuing low price over everything else and, as they say, knowing the cost of everything and the value of too little.

Up here....in rhododendron heaven........the box stores sell nothing more than the very common, basic hybrids. And nothing of substantial size. They cater to developers/landscapers and novice gardeners, not to serious gardeners/collectors or anyone looking for something even slightly out of the ordinary. And yes, they do sell at a discount compared to a specialty nursery, however you get what you pay for so in this case it is a limited and common selection and often, second quality plants.

I’m in the east bay, so this may be helpful since I’m local to you.

My closest Home Depot, San Ramon, seems to get a few different types each year, fairly good size and seem healthy. Prices are discounted. The thing is they’re not usually the right type for this area. But sometimes you can get a deal on a gem if you don’t read the labels and research a bit on your own instead.

As far as other big box stores, Orchard Supply (Lafayette and others), has more mature rhodies and they’re usually great types for the area, but they’re crazy expensive for what you’re getting. Also, still not a great selection, just a few varieties that are known to do well. Some may be types that grow large.

All the Ace Hardwares around me seem to have a great selection of healthy, mature azaleas, but not rhodies.

Armstrong (Dublin) has a good selection of rhodies during certain times of the year, like now. They’re healthy, semi-mature and some may be types that will grow large.

I’m planning a trip to Sonoma Hort in April. That should be a good place to find large types. :-)

My local nurseries are really expensive for rhododendrons. I have been using a specialty nursery by mail order, and that has pros and cons. The main con is I cannot see the plant I am buying and what I have gotten have been much more immature plants than I would like.