You can store the unused portion in the syringe in the freezer, that's the point of drawing it out. I do it all the time.
I'm pretty sure pharmacies will sell needles without a prescription, I buy needles from a US site (Medex Supply) to use with my animals and they don't require prescriptions for the needles or syringes. Even with IV drug users, it's generally accepted that it's better to give them clean needles because a lack of clean needles/syringes is not going to deter them from using drugs and the overall benefit from access to them far outweighs any risks.