Only use drop-shipping suppliers who are in business for a long time - ( a year or more), ask them how fast they deliver and most importantly see what reviews they get from their customers!!
Aliexpress is a site where you will find suppliers for most anything. You can even filter so that you can buy only 1 item at a time. This way you can sell your item first, and after your customer paid you, you go there and order it and let it be shipped directly to your customer! :)
I hope that helps.