Joann fabrics but they only have limited letters so u might find yourself short
Wal-Mart (in arts in crafts). A.C Moores, Michaels. I know has them but also check Home Depot and Lowes as well.
Babies r us near the bedding section
Babies r us has letters:)
I bought letters online at Joanns fabric.
Hobby Lobby and AC Moore has them. Also, make sure you go in your phone or online and print out a coupon. Honby Lobby usually has a 40% off one item and AC Moore has 50% off one item and sometimes 20% off total purchase. Hope this helps.
Walmart and Michaels should have them, I got letters at both of these places for my wedding.