Is it a list of records in a dropdown, or are you looking at a list of records in a table view?
For the dropdown list, you could add a new column where the user selects a letter from, and then put a valid_if constraint on the dropdown list such as
SELECT(table[column] , CONTAINS([column] , [letter selection column]))
If you’re looking at a table view, you could sort it by item name. Or you could add a virtual column on each row that has LEFT([item name] , 1), then group the view by that VC.
I’m sure there are more options, maybe someone else has a better one.