Most pet supplies stores have them. You can also go online to search for them. I did not find one that emits a beep, but there are a million or so that have the "static shock". The shock probably has a longer-lasting effect. I know that unless the mat is in place, one of the cats will venture onto the washer. And has been known to stand NEXT to the mat and pee over it!!

My washer seems to be safe (for the moment) but the chair in the sitting room is the new target. Fortunately I have it covered with a shower curtain. I may have to invest in another mat. Wish they weren't so expensive.