I see you're asking the best place where I can buy plants. I have several favorite sources, but I suspect you're more interested in where you can buy plants. Given that I don't know where you are, that's not an easy question to answer. I can tell you big box stores (such as HD, Lowes, and Walmart) often have a good selection of non-woody plants at competitive prices. If you're looking for non-woody plants, those are good places to get started. If you're actually looking to source shrubs, bushes, or trees (woody plants), I find local nurseries seem to have a better selection of larger and healthier plants. That's important to me because planting shrubs bushes and trees is difficult enough without having to deal with plant issues.
I'll say one thing, always check with a local nursery before you buy from a retailer. The Lowe's here in my town wanted $4.99 for a Bonnie beefsteak tomato plant, went to the local nursery up the road, beefsteak tomato plant, $2.98 and locally grown in Texas.