This has been discussed many times, there is solid scientific evidence that deer whistles don't work, they can't hear them, and even if they could, what exactly would they do? Probably jump right out in front of you!
My number one defense is to know the habits of deer. In general keep an extra eye out for them at dusk and dawn. Where I live the darn beasts fill the local streets waiting in line to eat rose bushes at night.
Take some time and study how these meat bags behave, it could save your life.
PS - 'deer alerts are an urban legend, don't waste your money.