Newegg is one of the most popular online electronics stores in the USA. Their top category is computers, which includes desktops, laptops, gaming computers, and refurbished computers.
You can get iPhones starting at $13.34/mo here. This is for an iPhone 7 (AT&T). They've got lots of different models priced at less than $25/mo. Most come with free shipping. Several are on sale with good discounts.
Huge selection of cheap refurbished iPhones with over 1,000 different offers. The cheapest one is just $35. They also have lots of unlocked iPhones and iPhones with plans for sale. You can get free delivery on most of these.