If you need a test here in Prague including for flying home again or to release yourself from quarantine you can book one at this link https://gendet.cz/ but you cannot pay online just cash once there I think. They also do mainly free tests for Czech Health card holders.
Choose the site very close to the studios Smetanovo nábřeží 198, Staré Město and it costs 1000Kc (about €40 or £35) You then just turn up and get the result via email (not sure if they send you a text to a non Czech number)
I used the same firm at a location near me at 10am and got the email and text result at 4pm. Very simple and good.