You already know that a lot of supplies at gigantic hardware stores have high prices. Remember that as you develop your overall landscaping plan, and as you integrate items both small and large into the project.
Sure, you can save on seasonal items by shopping at the right time, or by buying online. But on items from tools, to chimeneas, to decorations of all types, garage sales and flea markets can be your best landscaping friend.
You can take this concept even farther by keeping your eyes open for demolition and construction sites. Whether these are commercial or private projects, they'll often have piles of old bricks, concrete, usable lumber, and other items that are destined for the dump. You may be able to snag some of those supplies for free. Just be sure to ask a site supervisor before you start loading up your truck.
In short, the stuff that other people just want to get rid of can become inexpensive treasures that make your yard look gorgeous. But stick to your plan. It's easy to rationalize the purchase of cheaper used items simply because they're inexpensive compared to new. Do you really think that old wagon wheel will fit the theme of your yard, or are you just buying it because it's only $10?
So feel free to browse online classifieds and local flea markets to your heart's content. Just be sure you use your brain when it actually comes time to buy.