There are three possibilities of getting hold of mobile phone:
1) to buy, cost of the cheapest is from 4000 CZK (cca 100 USD); you should choose provider which you like (3 types):
2) to rent - telephone numbers for booking & shops:
a) Paegas - airport +420-2-20 11 45 02, North terminal of airport+420-2-20 11 55 21
prices: principal 4700 CZK; 183 CZK/day + 366 CZK
b) Eurotel - airport +420-2-20 11 66 38, Sokolovska str. +420-2-67 01 15 07, Pobrezni str. +420-2-67 01 14 60
prices:principal 5500 CZK , 7 days -1795CZK, 10 days - 2395CZK, 14 days - 3195CZK
3) to take your own mobile phone working on frequence 900/1800 (Europe, not US) and rent or buy SIM card, but you have to check whether your phone is not blocked only for your local provider.
prices for buying:
Oskar - Oskarta 400 CZK ( only 1800 MHz)
Pageas Twist cca 1300 CZK
Eurotel Go cca 1500 CZK
rent:
Eurotel Go - principal 500 CZK; lease 395 CZK for 3-30 days
Pageas Twist - principal 500 CZK, lease 366 CZK for 1-90 days