A SIM comes from a carrier/service provider. So you can get service. They can be purchased in a lot of stores (supermarkets, electronic stores, phone brand stores, etc.,) or you can also purchase them online. It really depends on which carrier/service provider you are wanting to go with that is in your area. You would get the card from one of those places, or the carrier/service provider and then get it activate by the carrier/service provider. NOTE: That will only work if your tablet is capable of having a SIM for such service. Not all tablets are.
Now if you are taking a SD card, rather than a SIM card, those you can still get a lot of places. (Walmart, Amazon, Target, etc.) You just need to check your devices specs so you can get...