Buying Temporary Pre-Paid Mobile Phone Service in Prague

Assuming you already have a smartphone, I would not buy a pre-paid phone:

• it is easier and cheaper to buy a sim or esim
• cheap phone will be limited
• if all you want to do is make phone calls and texts, which is essential imo but not all that frequent while traveling, I can easily do this with my current phone.
• if you need to do a lot of searches, run apps, store useful information, book or make payments, chances are that a cheap prepaid phone will not have the same CPU power or performance.
• cheap phone equals cheap camera equals lousy photos or videos. The camera on my pixel is excellent.
• Even if the prepaid phone had an excellent camera and CPU (more $$), it takes a lot of time to set up the apps so that your replacement phone runs as well as your regular phone.
• do you still intend to carry your regular phone with you on your trip? If so, more devices, means more charging, more things to fumble around with, more things that could be lost or stolen.
• I have never been to Prague, but imo that is not relevant to the question.

When I was in Prague several years ago I purchased a SIM at a local T-Mobile store, and they were kind enough to mount it in my phone (this was before eSIMs became popular). The problems came in getting the darn thing set up. It's not very common to find retail employees in Prague who speak English IME, and the SIM setup call-in system had its recorded voice in Czech. It was a difficult process for me, as I speak no Czech.

All in all, I would recommend getting a data-only SIM or eSIM before you leave for your vacation (I use Nomad eSIMs and like them) and use your US service for texting if it provides that service at no charge or minimal charge. Or use WhatsApp, which works on data and offers the same functionality as common texting apps (but the person you're communicating with has to also have WhatsApp). If you need to call back to the states you can use a Google Voice account if you have one at $0.01/min, and there's a way you can use Skype to call US numbers from abroad for a fairly low monthly rate. I haven't done the Skype thing, though.