Lily Ann Cabinets offers up to two 1/4 cabinet door samples at no cost; additional samples cost $8.99.
The samples are cut from a full-size door and measure about 5.5" x 7.5" in size, allowing one to see the detail of a corner, as well as the weight of the wood and quality of the workmanship.
Also available are full-size cabinet door samples at cost (couldn’t find the corresponding link via mobile view, but I believe it’s around $30), the cost of which is credited back when one places an order, as reported by @MrsGuin.
Shipping is free on orders over $2499; currently, all hardware is on sale at 20% off. (11/16/2017)