Once you start, you'll be hooked. No longer will going to the mall or box stores thrill you; instead, you'll haunt garage sales, auctions and thrift stores.
No farm will be safe from your prying, and you'll make your chauffeur roll his eyes every time he hears the words, Stop! I want to go and look in that flea market/estate sale/junk shop.
Never mind; once he sees what can be done with a few old paint cans, some wire and a bit of driftwood, he won't mind a bit - especially when he finds out that it's either really cheap, or (even better) FREE!
In time, he too may start to come home and announce that he has a present for you - it will be something from the recycle center, or that someone offered him when they cleaned out their garage.
Here are a few of the things that I've accumulated in exactly this way:
Chairs of all kinds are perfect in the garden as handy shelves for a display of pots and containers, or planted themselves.
It seems that everyone breaks the wooden handles of shovels and other tools; I bring them home, burn off the remaining wood, and use them in garden displays.
Enamel ware, especially the vintage kind, is great for displaying succulents in; the older and more rusted and battered, the better.
Vignettes of natural (or natural appearing) items make great little accents in a garden; sometimes, during a drought for instance, you need these bits of humor.
Keep your eyes open at garage sales, auctions and whenever you spot a junk pile. You never know when you'll see just the right old piece of junk. Think outside the box...
I spotted this rusted out kettle at the recycle center. Just because someone couldn't see the potential, that doesn't mean you should pass them up.
I'm a big fan of wheelbarrows, old watering cans and anything else garden themed.
I recommend that you shanghai lots of people to keep looking (and grabbing!) as much garden junk as they possibly can; there is no way one person can keep up with it all.