Just to give some additional options other then the ones already given and to add some additional info to another, My best suggestion would be to purchase from you local pharmacy a sharps bin. When you purchase one (and they are OTC, they usually have them near the pharmacy counter and you don't need anything special or anything to prove why you need to buy one, there are instruction inside as to how to dispose of them once filled. You can ask the pharmacy if they have one that you can dispose of some used needles as most have one since almost all are giving Flu shots so they most certainly have bins there that I am sure they would let you deposit into. Also most local First Aid Squads have a don't ask don't tell policy an would probably let you deposit them into their bins on their ambulances. Most if not all medical offices (local Dr's or Clinics), Methadone clinics, Alot of areas have needle exchange clinics especially in major cities. There are a ton of places. Just make sure your do dispose of them properly. The problem with disposing them in soda cans is that even if they are in the can they can still be picked up and gotten to fairly easily. So I would say just seriously look for a place to get rid of them properly right from the beginning. What state do you live in OP? I have been doing some searching for you, and it's very possible that your state may have a program where you can have a free container sent to you, and disposal services also free. Let me know and I will reply right away with some more info for you.
Best regards,
Pain