SIM CARD North-California USA

Hi friends,
I am California in the north for some weeks. I would like to use the Internet with a Sim card. (ATNT Wireless or T-Mobile USA, Tuyo, Fonic etc…) I can which card where buy. I know Starbucks and the hot spot. However, liked to remain flexible. Up till now found possibilities are very expensive this one. Somebody has experiences or can help me.
Thank’s ET