An easy source would be NAPA. They carry trailer brake shoes. They tend to be higher priced than a trailer parts store.
Or you could measure the drum diameter and width of the shoe and match one that way with other trailer parts stores. Look in the yellow pages. I would imagine, you would need to be near a larger city to get a store like that.
I live in Phoenix and use Auto Safety House. They have a website. www.autosafetyhouse.com.
And have a toll free # 1 800 352-5355. However there may be a closer source than Phoenix AZ.