Osmo sells lowercase/uppercase Words tiles exclusively on our website, and they are found in the add-on game section, under the name 'Genius Words':
They are $29 USD, and ship in a stackable storage container.
Note that when you use these tiles, letters in the Words app are displayed in both upper and lower case, as appropriate. (When you use our classic tiles, letters are always displayed in upper case).
Plastic lowercase/uppercase letter tiles are available for sale as a separate item to educators. Please contact us if interested in purchasing for your classroom.