I published a post about inexpensive and DIY sandpaper letters, with many ideas that work for sandpaper numerals as well as letters. Here I'll give resources specifically for sandpaper numerals and sandpaper-numeral presentations and extensions.
Had when i worked in a preschool and kids love them! I would use colored sand instead of white salt. DIY salt tray with alphabet cards. Easy to make and kids have fun smoothing out the salt after correctly making the letter