So, do you need to buy tile-sets from unity asset store or can you import them in, from you computer with out needing to pay?
And if it will need to be done with asset store, will it be free?
Basically Tilesets are just an array of smaller tiles combined to one large image. You can import your own images without a problem and crop them in Unity.
You can create your on Tilesets if you want. You can also use roalty-free one from the web or buy Tilesets.
It depends on your decision if you want to pay for tilesets or not.