I've gotten some of these referrals over and over by now from several peeps, so they must be the "go to" sources.
Plant It Hawaii concentrates on fruit trees. Their website is packed full of pictures and growing instructions. http://www.plantithawaii.com/ open by appointment only and also has a reportedly awesome semi-annual plant sale.
Big Island Association of Nurserymen, reportedly they have a semi-annual plant sale that is a must go to as well. http://hawaiiplants.com/
Hawaii Fruit Lovers Nursery for rare fruits, nuts, palms, etc. http://www.fruitlovers.com/indexen.html
This place looked interesting, specializing in azaleas and rhodies http://www.whitecloudnursery.com/
I saw this place on our last trip to the island and on the way to Volcano and thought that I'd like to stop sometime. Akatsuka Orchid Gardens and have also been told several times about Rozettes in HPP and about Paradise Plants down by Walmart.
I also want to support my local neighbors if they are trying to make a living or earn extra from their efforts. Here's one: http://eveningrainfarm.com/our-nursery just down the road from Sea View in Kapoho area.
My favorite way of finding plants are gifts from friends and neighbors, found at yard and garage sales, or found at farmer's markets. This way one can usually get lots of planting details and tips.
And when all else fails and you're trying to find inexpensive prices Walmart, Lowes and Home Depot often have good stuff.
I did read something, somewhere, regarding suggestions on making sure your newly acquired plants didn't come with unwanted pests, including coqui frogs, fire ants, slugs and I don't know what else - but I'm sure there's more. One suggestion was to rinse the plants thoroughly, or even put it in the shower and shower them up to a certain temperature. I'll have to research that more though, as I could see scalding tender tropicals or killing the plant. It's my understanding that its very easy to transfer fire ants as they can be in the dirt pots the plants come in, once they establish themselves on your land, it's quite an ongoing battle. The same with the frogs, them sneaky lil' buggers!