Which is the best cheapest network in USA for PAYG Sim


Ok peeps when i visit florida later in the year im thinking of getting PAYG sim cards, so which would be the best and cheapest one to go for to use for say a 2 week holiday. We have phones on 02, Orange, T moblie and 3 network but i have been told its cheaper to get a US sim card rather than use roaming or get bolt ons for use in the US. All phones are locked on these networks in the UK but not sure what happens if we use then in other countries.

Plus where is the best place to purchase ebay or elsewhere?

we tried the two way radios but well they are not very good as to many brits use them and not enough frequenceies