A sample bottle can be purchased from Fidelio in the Canifis General Store or from Rasolo the Wandering Merchant. It can be used to collect blamish snail slime.
The best way of obtaining a sample bottle is to go to Canifis and buy it for 10 coins from Fidelio. Although it resembles an empty vial it cannot be used for any of a normal vial's storage functions.
A Sample bottle is also obtained when using the strange potion on Sithik's tea during the Zogre Flesh Eaters quest.
Another sample bottle is obtained during Heroes' Quest. After greasing up the fishing pole a Sample bottle is what is leftover.
Sample bottles looks like an empty dye bottle.