Futon purchasing advice/suggestions, please...

After shuffling rooms around in the house, I now have a small office (9×12) that at the moment only has a desk in it. I would like to add something that converts into sleeping space, as I periodically end up with a houseful of overnight guests, and lack enough beds. I decided that a futon be my best bet.

Problem is, I went shopping this afternoon and none of the furniture store I went to still sell futons. I believe I am going to have to do this online. In looking at the various sites, it appears I am going to have to purchase the frame, mattress and cover separately. Is this correct?

I am looking for a wood frame, either in a dark mission or arts and crafts style, and some sort of contemporary pattern that would go with mustard colored walls with white woodwork, and black/white photos and and a few screen printed posters. Any suggestions as to where I should look? What are the considerations for buying a futon mattress?