1) First, of course, go to iStockPhoto.com
2) Once there, do a search (i.e. 'Luxury Homes', 'Oklahoma Homes' etc)
3) After the search results appear, you should see a Price Filter along the left hand side. Adjust the filter to either one or two dollar signs.
4) If you're having Agentfire purchase the photos and want to view USD pricing first, make sure to click the $USD tab when viewing the image. For reference, themes using regular width sliders will require size Medium, whereas themes using large width sliders will require a size Large