Can I buy an iphone and then a contract/sim separately?

If I buy an iPhone and amazon or ebay (these are cheaper ofc) will I be able to buy a sim (on contract) by itself for the iphone?

Yeah

Yes of course! All you need to do is to get on Ebay or Amazon.com & search for the type of Iphone you want. Whether it be a regular 4, 4s, 5, 5c or the 5s. Once you decide the one that fits your price range best, then pick the carrier you want it to be on. A lot of listings will notify what carriers the phone can be used on. Do not buy blacklist or bad esn phones, as they will not activate. A lot of carriers will let you use the Iphone on either contract or a pre-pay type.

Of course.

However, you need to think ahead though of what deal you will end up choosing and the network it's with. As you need to make sure you buy a handset that is either LOCKED to that network or just UNLOCKED.

Use a comparison site to find a decent sim only deal. There are thousands now, 30 day rolling contracts to 2 years in length! Try and weigh up the pros and cons of each. The longer ones will be cheaper but lock you in for longer. For example www.compare-sim-only-deals.co.uk compares just sim only deals in UK.

Hope that helps!

Actually the iPhone by itself is more expensive.

e.g. The 5s, 16GB
on contract - $199
off contract - $599
unlocked - $649

but to answer the question yes you can buy the iPhone & then the SIM afterwards. You'd just have to make sure the iPhone is unlocked or that the SIM you buy is from the carrier the iPhone is locked to.