The entrance to Namtok Sapan is in Ban Sapan, but a bit difficult to find as there are only very small signs. Following the small road to Ban Sapan at the beginning of the village is a smaller road going to the right. This ends up at a parking from where the jungle trail to the waterfall begins.
Although as waterfall Namtok Sapan is not spectacular but the walking trail to it is really very nice to do. But be aware that it is not a smooth walking trail and that you are really walking in the jungle. It can be slippery. And there are no shops to buy something to drink or eat, so take it with you!
Although Namtok Sapan is behind Khun Nan National Park it is not part of it and entrance is free. Walking to the waterfall takes at least about 30 min so be sure to leave about 2 hours before it gets dark in the jungle. There is no public transport going to this waterfall.
For more information about Nan province have a look at Destination Travel Guide Nan.