You can buy similar cards in drugstores, newsstands, convenience stores, etc. but they probably won't be quite as cheap. And as you noted it can be a challenge to tell them apart, as there will often be dozens of different cards for sale with a myriad of rates to various countries and lots of different surcharges and fees.

Unless you're making a LOT of phone calls, I wouldn't waste too much time trying to sort through them to find the absolute cheapest. Just make sure that the card you're getting is clear on the connection fee (ideally none), maintenance fee (ideally none, or at least not for a week or two), toll-free (i.e., 800 number) access surcharge (might be a few extra cents per minute), and rounding (ideally 1 minute, but if you're making mostly 15 minute calls, then a few minute rounding isn't a big deal). Most cards also have a payphone surcharge, so be sure to use your hotel room phone or some other regular landline and not a payphone.