Don't know which parking lot you will be in at Downtown Disney, but when you exit you will be on Buena Vista Drive, turn left and proceed to Hotel Plaza Blvd. (Hilton Hotel on the corner and right across from Disney bus depot). Turn right on Hotel Plaza Blvd. and continue to Apopka-Vineland Road (535) and take a right at the light. If you take a left at this light, there is a Walgreens maybe 1 mile due north. To go to Wal-Mart, travel on 535 south (turns into Vineland Road) towards Osceola Pkwy. You will pass under I-4 and thru light for International Drive. Wal-Mart will be on your right just prior to Osceola Pkwy. (toll road that goes east-west). Don't know what hotel you are staying at but you can re-trace your route back to Downtown Disney to return to your hotel. The store is maybe 15 minutes from Downtown Disney (depending on traffic). Should you need it, their phone number is 407-397-1125.
Downtown Disney is approximately 45-55 minutes from Port Canaveral (depending on time of day and traffic).