Originally Posted by AEmgee

Just to provide an update on this in case anybody else in the future has the same questions as I did:

1. I contacted MBTA and asked them to send me some CharlieCards. I never got any reply. However, about a week later I received CharlieCards in the mail. Unfortunately, the cards had an expiry date just a few days after I received them (and before my arrival!) so they were essentially useless.
2. The blue container near the Boston Design Center is a shop, but it is not open on weekends (and I was there on a Saturday).
3. I ended up paying $2.75 cash (no receipt or ticket given) to get on the SL2 bus and then transferred to the Red Line at South Station behind the fare gates without paying again, exactly as LoganFlyer said.
4. I think, but am not certain, that if I had paid the SL2 bus fare with CharlieCard I would have been able to further transfer to a local bus after getting off the Red Line within 2 hours for free? That would have been useful as there were torrential downpours at the time and I didn't spend more than an hour at Boston Common before needing to take public transportation to escape the rain!
5. I got a working CharlieCard from a friendly attendant at one of the subway stations. She had a whole bunch in her pocket. I used it for the rest of my journeys.

As an aside, Boston has some of the most interesting station names. I now know the meaning of "Alewife" and "Braintree"...