Reselling items is a great way to make money off things others are more than happy to get rid of. While you can, of course, peruse the free ads on craigslist for things you don't have pay for at all, but garage saling for your garage sale can be a really helpful way to bulk up your merchandise stockpile.
There is a certain brand of sale you should be looking for when getting things for your own garage sale -- the best kinds are the ones where everything must go! In these cases, the hosts are desperate to get rid of everything, and are more likely to have put things at low prices in the first place. Plus, if you ask for bargains on multiple items, they're more likely to go ahead and let you take them in bulk instead of trying to be nit-picky on the prices individually.
Even if you hit up these kinds of sales, you should still haggle! Bring your best haggling skills to the table and see if you can't negotiate ridiculously low prices. Remember, the less that you have to pay here, the more you can make when you resell it. Use the profit margin as motivation to be persistent about your bargaining; don't let them get away with charging you more than you're willing to pay.
Get items similar to what you already have, so you're essentially bulking up your stock. That way, you can advertise on your signs for the items you were already planning to advertise for, and you'll just have more of the same (and can say "HUGE SALE!" on your signs). If you get too many different items, you're going to have trouble picking a crowd to advertise to.
You need to know what you'll do with the merchandise if you can't sell it. That might mean making sure the stuff you buy could be turned into presents for your friends and relatives, or it could mean finding a place where the lot could be turned into a giant donation.
Now that you know what to get, check out GarageSaleCow.com for some local garage sale listings near you.