I asked a similar question a few months ago. I wanted to just go into a DVC Sales office to buy an add on contract since I happened to be in the Orlando area. I was told that I should not follow this procedure as it's much easier to call.
The advice I was given held true. I used chat on the DVC website to have a Guide assigned and not more than 15 minutes later I was off the phone with my add on contract purchased.
For your question about the blue membership card: No, you cannot get it in an office. It has to be requested online or through MS. A blue temp card can be printed from your dvc account page... its unlocked less than 2 hours after purchasing your direct add on contract.