The best price I could find for straight black 3" letters and numbers was Wal-Mart. They had complete sets with 4 of each letter and numbers for under 10 dollars. They worked fine and were a quick and easy fix. Once I repaint my boat I will order a much nicer set from iboats, they have great lettering sets.
I had good luck with gulfstream for my lettering, the boat name and other graphics. They are not the cheapest but the quality is superb and they are very easy to install. Plus - you can design things however you want to online and see what it will look like