What to do when manufacturers....

This depends on your present track record for sales of this item. If you're already up and selling this product and having it made by a rival manufacturer, you can produce sales records to demonstrate that you sell x amount per month.
Once the potential "new" manufacturer sees you sell so many hundred/thousands every month on a regular basis, they should be more amenable to running up a small trial batch for you.

If however you are just starting up, it may be that you have to go and see someone, preferably a decision maker, in their company and show them your business plan, complete with projected sales forecast.
If you don't have a business plan, you'll have to work that much harder to get them to take you seriously.
If you can convince them that you know what you're talking about, they may consider doing a small trial order, although you may have to pay a higher unit price for it, as I doubt they'll risk incurring a loss on a "newcomer".

Almost all the seller provide you samples
I am in china and i know almost all the seller in China can provide samples for importers. Some of them request a min order but usally not a big amount .

Size of Order
HI to the first floor,

Well,you could make a small order to the manufacture.but they will offer you a higher rate,but it is fair to both of you.