All you have to do is ask! Decide what type of items you want to have at the auction, and contact the businesses, either by phone, or in person, which is preferable, if you can. Here’s a sample script to build on:

Hi, I’m Name, and I am calling about a fundraising event for Whatever Fraternity to be held on Date. We’re trying to raise money for whatever purpose, and I’d like to know if you are able to provide an item or service for our silent auction.

Be prepared to follow up with a letter (on organization letterhead) stating the purpose of the fundraiser, and how the donor will be credited at the event.

Local restaurants usually are pretty good at providing meal certificates, and nearby cultural institutions will usually provide memberships, as might gyms, spas, and local artists/artisians.
Think of some other businesses that might be interested in promoting themselves to the fraternity.

Have fun!