You have to go via a resort. There are no buses to DTD from any theme park or from TTC.
From Epcot, the best place to transfer is POFQ. From 10 a.m. thru 11 p.m., you can also ride a boat from POFQ to DTD, but it usually takes about 10 minutes longer than the bus.
Or if you don't mind walking half a mile, you can take the bus from Epcot to SSR and exit the bus at the Congress Park station (the 4th stop at the resort). Then walk straight back between the first row of buildings, turn left, and follow the shoreline around to DTD Marketplace.