If you are looking for one in the hundreds, Walmart, but if you are looking for one for about thirty bucks, ebay... but sometimes you get bid up.
If you are really looking for a place to find a cheap IPhone 4 whether the IPhone is used or brand new, then you should definitely check out the apple store.
Right now, you can get a cheap iPhone 4 from Sprint and AT&T. You can get one from AT&T for around $100.
WAL-MART
I want to suggest Gadgetzkart as the best iPhone case at affordable prices. They provide the best quality iPhone case that helps to protect your iPhone from all sides. They have a lot of collections of iPhone cases in different designs and colors.
The OS 8 is for the iPhone 4. The iPhone 4 is a really popular phone.
eBay would be your best bet
It really depends on your preferences. The iPhone 4 might have some more improvements but they are really similar.
FAMILY DOLLARS www.Cheapipodssale.com is best place to find cheap iphones with huge collection.
you can't unless someone stupid enough on ebay or amazon makes you get it cheap
Some sites that sell iphone 3gs cheap are ebay, craigslist, or check your local listing. Usually there are people who are giving things away are selling them for really cheap.
There is not really a way to score the Iphone for cheap as the demand is still very hot so they will sell regardless. You can try and get one on Craigslist or Ebay for a reduced rate as they will be used.
From a robber who had his share with an Apple store.