My family - How many of you cannot name someone in your family that has a house full of stuff? Most don't know what they have in the back of the garage piled in a wall of boxes from the 1960s. My mothers have lately been cleaning out house and want to get rid of it. I accept it and keep what I think is useful and freecycle or donate the rest. It also pays to ask a family packrat if they have something you are looking for. I remember telling Mom-in-Law how I was starting to look on Ebay for an oval bubble glass convex picture frame for my bathroom decor, and Viola! She pulls out two from the closet!
Freecycle - Join your local Freecycle group. Here you can offer that stuff from the back of your own garage and get for free what others are giving away from theirs.
Thrift Stores - I love thrift stores. Especially one of my local ones that has free day every quarter! Thrift stores are good for when you have a clothing need, it's the best chance to actually finding a used pair of jeans to fit you that you would wear or some other specific need you have. Also, on free days or in the free boxes, I take what I think may become useful down the road.
Flea Markets - Some flea markets are good; some flea markets think a rusted spoon is vintage and charges $10 for it. Always check out a flea market and see how their pricing goes. Sometimes you can find good ones close to garage sale prices.
Auctions - Around here there are tons of auctions, but always have an idea of what stuff costs. In this area they tend to bid very high to even paying more money than it is worth. Once I saw a pyrex dish I wanted and inspected it and found the grocery store price tag of $8.99 on the bottom of it. I figured I would pay $5, it went for $12!!! If I am not sure of how much it is worth, I make myself stop bidding way under what I think it is worth because I don't want to feel like a dweeb when I get home and find out I overbid. Definitely go on rainy days, snowy days, bad weather days, days when there are many auctions advertised or a major event is happening in the area; if you hit a day when not many people are out, you can get wonderful bargains!
Clearance Aisle - I always check out the clearance aisle of stores I frequent. They usually aren't cheap enough to beat the price of an item at a thrift store, but things like vacuum bags will never be found at the thrift store.
Garage Sales - With gas prices what they are, I do not do this as often as I used to. I tend not to stray too far off my normal beaten path, because a 5 mile trip down a backroad to a garage sale that offers nothing of interest is too much of a waste of gas. I try to hit those that describe themselves more as rummage, city-wide or multi-family sales. Those held by churches are generally always good.
My own backyard/garage - Leftovers from previous owners or my own packratedness comes in handy. Organize it as best you can so you don't go buy something you already have. Keep old clothes and grocery containers, etc. My rule is, I save it for a number of months, if I still can't think of anyway I could possibly use it, I throw it out.
Online - Looking on Ebay, or search engines to compare prices can sometimes find you a better deal than buying it from your local store, but not always. I always factor in shipping as part of the price. If you are looking for used books, use BookSpot to compare prices. I find the internet most helpful for buying larger purchases. Last week, I bought a new printer for the price of a lesser quality one just by doing extensive surfing on the web - found it on sale with free shipping. Also, using the internet to research can keep you from wasting your money by buying an inferior quality product. I always look for customer reviews. I was tempted to buy the inferior quality printer at my local Walmart, but luckily, I was patient enough to go home and look it up and find that many people were unhappy with it.