All the major quilt fabric companies regularly donate items for guild activities.
Also the batting companies, notions, book publishing, thread companies, etc.
But you're going to have to write to each - addresses/emails are on their websites.
You can same the basic email/letter to all of them.
Some sites have a separate email for donations, otherwise you can write to the main address.
Supply all the information about your guild show, including the dates and location, and the address to ship the products.
Your guild can supply a reciept for the donation as well (that's usually sent after the items are received, but mention it anyway).
It would be best on guild letterhead or from a guild e-address.
Write to three or four of each category - that way you're sure to have a nice selection of prizes.
Don't write two weeks before the show, give them plenty of time to send a package.
And write to more companies than you need because you might not receive from all of them.
It's not hard, but does require some work and time on YOUR part.
And don't forget to follow up with a written thank you from the Guild.