In order for you to get a SIM card, official AT&T policy is that you have to have a current AT&T account. Under no circumstances are they obligated to give you a SIM card if you don't have an account. If it were me, I wouldn't care. Just tell them you just need the sim card and are going to activate that sim card with customer service later. Tell them you are activating a new account and customer service needs you to have a new SIM card. Since there's no way for them to verify this you can get away with it. Since you already have a phone, there will be no contract involved at all.
If that doesn't pan out, retailers like Wal Mart and Radio shack can sell you a plain AT&T sim card for $26 with or without an account.
Keep in mind tho that just having a sim does not necessarily give you service