Yeah, if you want to go with AT&T, other than online, the best place to get a SIM is at an AT&T shop. Unlocked phones and prepaid service aren't big here, which is why you can't easily find SIM cards sold by themselves.
As for Straight Talk, even though I use them, I probably would not recommend them because of their hidden data limits. Plus, I think the Straight Talk SIM is only sold online, and I don't think they ship outside the US. It costs $15, but if you look on eBay, those who ship internationally are charging more just because they can. Plus, if you need customer service, their customer service is mediocre at best.
H2O Wireless is an MVNO that uses the AT&T network. In addition to being sold online, I think SIMs can be found at Best Buy. I'm not sure if they offer a microSIM. Right now they're offering 30 days of unlimited talk, text, and 500MB of data for $50 plus $20 worth of international calling (you have to check the rates on their website).
https://www.h2owirelessnow.com/pageControl.php?page=intCalling
The AT&T MVNO that I hear is best is AirVoice. They also offer 30 days of unlimited talk, text, and 500MB of data for $50 but only $10 worth of international calling. Their international rates seem to be different than H2O. Their customer service is the best of any AT&T MVNO, as their customer service is US based, not outsourced. The problem will be finding a SIM. I don't think they're sold at any large US retailer, you may only find them at small 3rd party wireless dealers. It may be better to buy it online from eBay or something. Also, I don't think they offer microSIMs.
http://www.airvoicewireless.com/PDFDocs/AirVoiceFreeILDRates.pdf
Something to keep in mind is activation. For the two MVNOs that I listed, you probably will either need an internet connection or phone service of some sort (to call customer service) and activate it. Also something to think about is the refill card. H2O you can find these at Best Buy, but with AirVoice you'll probably have the same problem as with finding their SIMs. Also, with these two MVNOs, once you run out of data, there's no way to buy more.
Overall, AT&T Prepaid is probably the easiest, since there are physical stores you can go to, and you can have the sales rep take care of everything in-store.