I really love container gardening. I like knowing exactly what type of dirt is where, what plants belong and what mysterious plants don't. When I first planned to make raised garden beds, it wasn't just for some usual reasons: not having to bend over so far, gopher proofing bottom layers with wire mesh, keeping track of past and future crop rotation placements, etc. It was because I really have very little idea of what's a weed and what's not a weed until it's got a few leaves on it. There's no way I'd just plant a few seeds in flat dirt in rows and keep the weeds from starving the plants I intend to grow. So I'm always on the lookout for new container ideas.
Maybe after I'm a bit more experienced I'll be more comfortable throwing seeds into miniature ditches in the ground, but until then, I'll stick with containers. From paper rolls and empty yogurt cups and old cartons, to free nursery pots and scrap wood screwed together randomly to make all shapes of boxes, free planting containers are everywhere! Mostly I find them in the grocery store in the shapes of plastic food containers that are easily cut with a utility knife, and on Craigslist.org where there are people almost every day giving away free pots of all sorts. Sometimes they're just moving, or cleaning out a shed. Other times it's free nursery pots overflowing from a landscaping company - or even a nursery itself!
The big containers (raised garden beds) I have so far in my very small backyard were all put together using free scrap wood. So long as the wood isn't treated or painted on the inside with old lead paint it seems to be okay. I used all sorts of screws, and nails, basically whatever I found laying around or super cheap (or free) on Craigslist.org. If I get free wood with screws or hinges attached, I just undo them and save them for later. Mason jars are a great way to store all these things without fear of rust or getting poked through a plastic or paper bag.
I have found dozens and dozens of free pots online also. I try to keep looking only very close to my own neighborhood, otherwise the price of gas defeats the purpose of finding free things. Ordering from Amazon.com and getting free shipping is sometimes a much simpler option for the same cost as gas spent driving around picking up little items here & there. But I'm getting better at being patient and more frugal each week. I've learned to look for larger quantities of free items at a single close location, or finding things "on the way" so I don't have to make multiple trips. So far, so good.
If you're going to be searching on craigslist, too, try looking for more than just "free pots". Sometimes people use different phrases in their free item titles and descriptions such as: nursery pots, plastic pots, plastic planters, hanging planters, half-barrels, seedling containers, etc.
If you're not sure what size pots, planters, or containers you should have for whatever you're growing, check out this extremely useful factsheet: http://ohioline.osu.edu/hyg-fact/1000/1647.html.