You'll need to take a taxi either way and this place (the bus station): Mrągowo D.A, is the easiest place to get one.
Are the bus tickets bought directly from the driver or in a machine at the station?
Depends. No machine, but if there's a ticket office in the bus station, buy them there, or otherwise buy from the driver. Try to have the correct change or something near it (i.e. no 100zl or 200zl banknotes).