Where can I buy an iphone 4s for under $300?

I mean an actual store not an online site

Craigslist or ebay

No actual shop will sell iPhone 4S for under $300. If they do, the phone won't be new. So your best bet would be eBay / Amazon / Craigslist. You won't get new though in your budget.

I don't think you can get it for under $300. Unless stolen and sold on ebay.

Try best buy