Amazon offers Prime members the Trademark Global Kids' Alphabet Foam Floor Puzzle Mat, model no. 80-31515, for $21.67 with free shipping. (Non-members can spend $35 to qualify for free shipping; otherwise, shipping costs $7.27.) That's the lowest total price we could find by $8. It features 26 water-resistant interlocking tiles with pop-out letters.
Note that the color may vary.