Information Please,re Pay as you go Sim cards

Posted by George Pye
I am visiting the USA this summer on holiday, and I use as my personal phone an old Nokia GSM, with a UK O2 (Cellnet) pay as you go Sim in it.
Rather than paying expensively to ring the US, (or for my kids to ring me), I want to buy a local pay as you go SIM, and use it in the US.
I will be travelling in CA, AZ, UT and NV, and want some coverage in those areas (I am aware that some of the time there will not be any coverage). Are SIMS easy to get hold of? for instance at LAX, or at shoppin malls? Help please