Where can I get a cheap iPhone 6?

Any ideas I need one for this week

Simple: you don't.

New iPhone prices are pretty much set in stone by Apple who leave retailers with very little profit margin to play with so there's no genuinely cheap deals there, and used iPhones are commodities with very predictable pricing according to the phone specs, overall condition, whether boxed with all accessories or not, and whether it's factory unlocked or subsidy locked to a particular carrier.

So your cheapest one will be a used and abused broken non-functional example, and the most expensive will be a nearly new mint condition, unlocked, boxed 128GB version. But you can find fully functional examples which are cheaper than most simply because they've got a lot of cosmetic damage and are locked to whichever carrier you use.

Be very wary of buying them privately. It's incredibly common for dishonest sellers to sell them and then report them as lost or stolen to their insurer, after which your new phone gets IMEI blocked and becomes permanently unusable. Reputable retailers will offer guarantees against such dishonest dealings and will usually offer a warranty on the used phone as they will have fully tested it pre-sale. Here in the UK that warranty can be a full one year, just like on the new phone, from suppliers such as CEX.

EBay