Be a Bulk Buyer
Hardware superstores are wonderful in that they carry a tremendous range of landscaping supplies to suit your needs at a moment's notice. But you will often pay a dear price for the convenience.
As evidence, you can evaluate prices for common products, such as mulch. Mulch is an incredibly basic and simple material, and it's easy to pick up a bag or two at a time when you're gathering supplies for your projects.
But those individual bags are enormously expensive when you stop to add up the numbers. Local landscaping companies often drop truckloads of the stuff right at your home (minus the fancy bags) for a much lower price. What's more, some cities give away mulch for free, so long as you have a vehicle to transport it.
The same logic applies to other landscaping items. That includes sand, gravel, straw, wood chips, and much more.