Hi Margaret, washer sizes are normally designated by their ID and I doubt you'll find any advertised as 13.5mm, if you mean ID, then a 1/2" one might be about the right size, but you may just have to file it to fit.
M6 Form C Washers [BS 4320] have an OD of 14mm. You could easily spin one of these in a drill chuck against abrasive paper to get it down to the required OD.