My husband and I looked around at mattress stores (Mattress Warehouse, etc.) and after finding the kind we wanted, bought it online from Costco for under $600.
I would highly recommend doing it this way. Whether you find the brand/type itself or narrow down what you want in terms of firmness/features, it is helpful seeing the mattress options in person, then you can shop online for the best price. We liked a firmer Sealy mattress at the store, left, then went on Costco’s website and found the same thing for a lower price, including a box spring and delivery.