Generally, a charity auction is composed of free things -- donations from companies as well as individuals -- that are then sold for money to donate to an organization of some sort. But how does one go about getting free items to auction off? Outside of asking friends and family to pitch in with goods from their workplace, there are ways you can approach professional companies and ask for donations of both materials and events (perhaps in the form of tickets or passes).
Approach manufacturers directly. Trying to talk to the middle man is usually pretty futile, especially if it's over a product that comes at a much lower cost when purchased directly with the manufacturer. Track down the company itself and do what you can to get in their good graces -- be polite when you converse with them on the phone, send them letters with gentle reminders if they made any promises to you, and don't be pushy about it.
Target local companies. Giant national companies may be able to give away a lot without it impacting their profit much, but they also don't have as much vested interest in helping out your community. Find larger local companies who have plenty to give but also plenty to gain from doing so and go to them for donations first. Explain your purpose and tell them what your charity auction is for, and hopefully they can give you a few freebies, even if they're small.
Sign up for sample offers individually. This is a painstaking way to go about putting together a goodie basket, but it won't cost you anything! Many companies offer samples on their sites for which you can sign up, but these are almost always limited to one per household, and you won't be able to get more than one unless you specifically explain your cause to them (even then, it's easier not to try the actual sign-up for the freebie offers and just call the company directly).
Send thank-you letters afterward! Nowadays, this common courtesy gets lost along the way, but if you send thank-you notes to people who have donated, you'll be surprised by how much more happy to help you next time they'll be! It takes just a few minutes to write down a few lines of gratitude, and the cost of one stamp will be far surpassed by the connections you'll make.
If you don't like the auction idea, try gathering donations for a charity garage sale, which you can list for free on GarageSaleCow.com!