If you want to purchase a new sim card, they are $10 each. If you are going to purchase a new device that uses a different sim, then the sim will come with the phone so you don't have to worry. But there is so mini, only regular, micro and nano.
You can always cut it on your own, that's what I did & works great.
As a bonus if cut properly you can keep the outter frame so you can swap between devices depending what size it needs. Personally using the same SIM I go from nano-to-micro-to-standard