There are no buses between Epcot, Magic Kingdom Park, Disney's Contemporary Resort, Disney's Polynesian Resort or Disney's Grand Floridian Resort & Spa due to the Monorail System. To travel (directly) between these destinations the monorail is the only option with transfers at the Transportation & Ticket Center when applicable.
MK -> Epcot: Express Monorail to TTC, transfer to Epcot Monorail
MK -> CR: Resort Monorail, 1st Stop
MK -> Poly: Resort Monorail, 3rd Stop
MK -> GF: Resort Monorail, 4th Stop
Epcot -> MK: Epcot Monorail to TTC, transfer to Express Monorail
Epcot -> CR: Epcot Monorail to TTC, transfer to Resort Monorail, 4th Stop
Epcot -> Poly: Epcot Monorail to TTC, transfer to Resort Monorail, 1st Stop or Epcot Monorail to TTC, then walk from TTC to Poly
Epcot -> GF: Epcot Monorail to TTC, transfer to Resort Monorail, 2nd Stop
And depending on the times of EMH, it may be completely possible to just use the monorail.