You should be more specific about the overall size letters because there are many variables. It also depends on how simple you want to make your letters. I tend to prefer Block letters or All Cap Block letters/numbers because they keep it simple. For inexpensive, nice looking, and EASY, the Wilton products are great.
Personally, as a hobby baker, I endorse these:
Although discontinued from Wilton, these are available on line at various sites:
For tappit style letters, the Windsor ClikStix are wonderful, but they are also more pricey.
If you purchase the Windsor (I recently purchased the ones listed below after a very informative presentation by a member at the San Diego Cake Club. Pay attention to the size and have a ruler near your computer when/if ordering online.
OR, you may wish to get the Groovy Upper/Lower/Number sets. They are quite nice. I ordered both the Block upper/lower/number and the Groovy upper/lower/number.
I use FMM Tappits (Script) for writing on cakes and cake boards. I also use Alphabet Cookie Cutters, when I want the letters to be large enough to stand on the cake. I got both from NY Cake. Fondant Source has better prices. I also have an alphabet chocolate mold that I use when I want a large letter size. I'm a hobby baker so having the 3 sizes to choose from works for me. HTH