Great idea. Hershey's does customized chocolate bars. I'm not sure what sizes they have but it is sure to be on their website.
http://www.winbeckler.com/custom_candy_molds.asp
http://www.customsweets.com/
or you can get a bar mold and a number/letter mold and do them yourself....just use a touch/brush of warm chocolate to attach the letters to the formed bar
http://www.candymoldcentral.com/scripts/prodList.asp?idcategory=1653&curPage=3&sortField=sku (page 3...look on through...there's some really nice letter molds. also, there's numbers in different sizes)
oh oh oh...check this out!
http://www.yournameinchocolate.com/large_rectangle_chocolate_molds.html
http://www.customsweets.com/holiday_mold_designs.html (image at top shows customized small bar with a date)
Boy who knew there were so many cool chocolate mold places!! 
this was actually my mothers idea since she figures i might as well show off to my family members that live in KS who i havent seen in five years and to my cousins from canada that i can do this and that my education in pastry arts has paid off and the fact that i will most likely will be doing the wedding cake for my sisters and her fiancee's wedding on top of being the maid of honor for my sister in the wedding
There's a wide variety of ways of doing it without paying a high price for a custom mold....mold the bars, pipe out lettering by following a pattern if necessary, attach the lettering to the bar. If you're good at writing, freehand the message on the finished bars. Make your own mold. Let Mama buy the molds (it was her idea afterall). Depends on how much time and patience you have to put toward the favor aspect of it, eh?
Quote by @%username% on %date%
%body%