Many parents search for piano sheet music with letters to relieve their kids of the pressures of reading music. There are good reasons for this: reading music is inherently hard and kids are often left confused and unable to enjoy playing. Hence the search for other "languages" for music besides musical notation, such as letters, numbers, and colors.
There are a few products that offer letters as a substitute for notes. It is very likely that you will have to put lettered stickers on your keys to give the child a visual reference point in order to make use of these methods.
The time honored tradition is to simply write the names (letters) of the notes directly above the notes (on the musical page) in question, giving the child at least a clue to the location of the note.
One advantage of this system is that it teaches the child the names of the keys (letters) as well as the physical sequence of actions required to play the song.
But there are disadvantages. Suppose your child is too young to know the alphabet securely? Letters are a little more difficult for children than numbers, as numbers are almost second nature to all kids.
Before selecting a method of avoiding reading music (numbers, letters) you might do well to rationalize for yourself why you are doing this.
The only reason to avoid reading music is that it is not pleasurable except to experts, and becoming expert is beyond a child and almost all adults.
Yet, the impulse is there to play music, just not through the medium of musical notation.
So your question should be, "What is the best way to avoid reading music?" rather than "Where can I find music with letters instead of notes?"
Step back a little and think about this: it is easier for a child to substitute numbers rather than letters for the notes of a song.
This is partly because numbers are usually learned much earlier than letters for a child, and thus are more firmly entrenched in their consciousness.
Numbers also have greater relevance to music theory, since numbers are the same as the classical intervals, and are used in chord symbols and in other ways in musical notation. Letters are used as well, but kids often become confused in the sequence of letters when taken out of context, but numbers are indelible.