First of all you will need to buy what we suggest for our product. They are not very common stuff, like a charger for mobile phone, you can buy in any store.
But they are quite common you can find easily on Amazon, eBay or AliExpress.
Where to buy these stuff depends how long you are available to wait.
A Chinese site like AliExpress provides you the best prices, but often you need to wait for one month to have your stuff at home. Using Amazon you have a better time to receive stuffs, but the price is not so cheap as you can find in AliExpress. For eBay it is variable, depending on the reseller.
We will suggest possible packages or versions so it will be easier for you to find the exact product on the market
Amazon: http://www.amazon.com (or take a look to your local version)
eBay: http://www.ebay.com (or take a look to your local version)