Alice and the Mad Hatter appear at breakfast only at 1900 Park Fare in the Grand Floridian. You can make reservations for this meal
HERE or by calling Disney Dining at 407-WDW-DINE.
The Alice in Wonderland character may also be present in other areas of the parks. For example, Alice is sometimes at the UK pavilion at Epcot and Tweedle Dee and Tweedle Dum sometimes appear near the Teacup ride in the Magic Kingdom. To make sure you find Alice and her friends, you should go to the Guest Relations office at any of the
theme parks;
these are noted on the park maps. Ask them to call the "character
hotline" for you. The character hotline will be able to tell you where Alice, or any specific character will be in the parks on a particular
day.
As for merchandise, the mix of items sold is constantly changing. However, generally there is some Alice merchandise sold in the parks. The largest stores at WDW are the World of Disney store at Downtown Disney and Mouse Gears at Epcot. If Alice merchandise exists in the parks, the folks there will be able to help you find it.
Have a wonderful visit to Walt Disney World.