Kauai Nursery and Landscaping is centrally located near downtown Lihue. They have a huge selection of plants, randing from the plain (slabs of grass) to the tasty (oranges and limes) to the exotic (plumeria trees and lush ferns).
If you're looking for a wide selection of vegetation, Kauai Nursery is the place to go. They do not have exceptionally low prices, and some of their stock is a little pricy. Unfortunately, their customer service could be improved.
If you are visiting from out-of-state, Kauai Nursery can ship your plants back to your home. This comes for a price, but where else can you get the plants and shipping conveniently combined?
I have not personally tried their landscaping services, although many commercial businesses on Kauai use them a lot.
PROS: Wide selection of plants, conveniently located near downtown Lihue
CONS: Mediocre customer service, average pricing