I have also been using them for about 4 years and have actually 'converted' a few of my girlfriends into using them. I love them. Haven't used anything else since.
My town had a fair about 2 months ago and a vendor was selling in pack which had 3 different sizes. One regular size, one mini to keep in your purse and one big one which he said is for calluses. All of these for $25, I have to tell you that using the crystal for my feet has made a huge difference as well, so if you see it, snatch it!!