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Here's a link to the sims on Amazon AT&T MicroSim Starter Kit Plus 2FF/3FF Adapter for GoPhone Devices (No Annual Contract) [link]
You can also get them at Best Buy, or any AT&T store. If you go to a corporate AT&T store the Sim is free, just pay for your first month and they'll activate it for you there.
This is the one I bought from amazon. You can use it for post paid and prepaid.
AT&T 3-in-1, Triple Cut Universal SIM Card Starter Kit for GoPhone Devices (No Annual Contract) packaging may vary [link]
Just go to Walmart/Best Buy/Target/etc. and buy a GoPhone SIM kit - it has a tablet option that should work for iPads. You should be able to set it up by inserting the SIM card and going to Settings > Cellular > Manage AT&T Account... (you might need to be connected to Wi-Fi the first time you use it).
Amazon also sells it: [link]
I've decided to buy a "pre-paid" AT&T sim from Amazon. Here is the link: https://www.amazon.com/AT-Universal-SIM-Contract-packaging/dp/B00I1W6FDA/ref=sr_1_6?keywords=at%26t+sim&qid=1565966770&s=gateway&sr=8-6
Can anyone tell me if I need to activate the card to pull the OTA or can I just stick it in my phone and get the update right away via Wifi?
would buying an att gophone sim work to upgrade?
You can get them from Amazon: