I generally ask directly.  My children's schools have annual auctions/fundraising parties where items get auctioned.  I ask the businesses I regularly support (restaurants, hair and nail salons, toy stores, etc) for donations.  Most businesses are happy to donate a gift card, since it brings a new customer in.  I also donate a cosmetic service from my dermatology practice for the same reason.

If a business I visit weekly (such as a nail salon) refuses, I am offended and typically stop patronising them. It shows they don't value me as a customer.