Ana,
You are very welcome! I am not sure what that person was talking about. I have never heard of any of the metro stations not selling the metro card or running out of them. Yes you go to any of the brownish machines with all of the metro info on them. One of the options is to get a card. I am not sure if you press it twice but it is fairly easy to navigate. If you are landing at Reagan National at 10ish-2 pm, head to the C Terminal Concourse info desk. I should be there and would love to and gladly help you out. And even will head to the Metro Station to help you out. Hopefully not flying Southwest or one of the A terminal since it would be a long walk/bus ride.
I suggest you start out with 10 dollars on the card. If it goes low, you can add money to it at the metro station, any of the buses and some cvs'. From the info you mentioned here is the trips you would take:
Crystal City-L"Enfant Plaza on Yellow line for Air and Space Museum, either hop back on the train( Archives), walk or take Circulator bus to National Gallery of Art. After visiting the National Gallery of Art, take Circulator bus towards all the monuments. Since the monuments and memorials are pretty close to Foggy Bottom area, I highly suggest you head to Georgetown for dinner. So take the Circulator bus towards to a metro station that will be an easy transfer to the Georgetown area or Circulator bus to Georgetown. Coming back, either take the 38B bus towards Ballston or the DC Circulator to either Rosslyn or back towards downtown DC. For the 38B bus, get off Rosslyn and then get on the metro blue line towards Franconia-Springfield. If you need anything else, don't hesitate and send me a message.