You can order it online in the Apple webpage and have it shipped to your home! (it would be the No Contract T-Mobile one if you want it Unlocked)
or..
You can try to find an Unlocked one from eBay.
Prices will be a bit higher if you search well or WAY higher if you go for the first result after your search.
Be sure to look at the sellers rating and always read the description of the article and the conditions of it. I only buy from people who have more than 100 positive feedbacks.
If you want to feel more comfortable of your purchase, you can talk to the seller and ask for the IMEI of the iPhone to check it with
Apple's own tool to check device Activation Lock status (in case you decide to buy a Mint-condition used one.)
Hope this helps.