Ahhhh, the hard supply to get! Well, I'd start by doing a search on this site - for the mp4 player on the home page - you can even email Salehoo - and they'll send you a list of websites that sell these.
When or where you decide to order, I wouldn't order but a few, if they'll let you ---post the site here - you are thinking of using first. Maybe someone here has had an experience with them. Pretty websites don't always mean they come through with what you expect.
Just send them an email and ask can you order 1-2 samples? And if it's what you want, or what you expected, you can move forward from there. You can check the frequently asked questions on most websites - they will help you some.
Good luck - post if you have more questions. I'm sure you'll get lots of replies. I'd even do some research on Ebay for that product to see how well it sells first.
Where were you going to sell your products?