Button To Jump To Index Location In Combo Box.

I've been using a combo box to jump to a specific record in a form. But, even with the comb box set at 15 lines long, with many hundreds of records in the form the vertical slider on the combo box is a little small and can be jumpy (moving 1/32" jumps 10 names, etc.). The combo box is formed from:
SELECT [cust num], [last name], [first name]
FROM [this table]
ORDER BY [last name], [first name];

I'd like to put in some boxes that will jump the combo box to a certain set of names. For instance, put in a box with a label on it that says "M". Pressing this button would jump the combo box to the first [last name] that starts with the letter M. For instance, if I have records for:
Lupold
Macaroni
Martin
Then it would jump to Macaroni. But, if Macaroni is deleted, then Martin would be the first name that begins with M and pressing the "M" box should jump the combo box to Martin.