Where can I buy a cheap platform bed frame
Goodwill or you can check craigslist.
You can purchase cheap platform bed frame from online sites like Amazon through http://www.amazon.com/gp/search?ie=UTF8&keywords=white%20platform%20bed%20full&tag=recommended0b-20&index=garden&linkCode=ur2&camp=1789&creative=9325. You can as well get it from K-Mart and Sears at good cost.
You can surf the Internet search, a lot of brands, after comparison you will get what you want, I wish you good luck