Put them in a sharps box - we have a proper one in one handling set up, but a homemade on in another, which is perfectly legal and adequate - use something like an old ice cream tub, cut a wee slit in the top, tape the lid to the box and put on a large label saying SHARPS and that's fine. Depending on how much business you do with your vet, they may charge you to dispose of it when it is full, but there isn't really another option, I don't think. When my mother-in-law lived here, she used to take the real one to the doctors and get a replacement - she did that on a regular basis anyway as she is diabetic and they didn't seem to mind one extra once a year or so.