I buy shared proxies from proxybonanza.com - 6 bucks a month for a short list. Fast, work perfectly. Just enter your password and off you go.
Can you select what part/state of the U.S. the proxies are from?
i have 20 private proxies that im not using at the moment. Name your price.
"Free" proxies can be a good thing, it just depends on what you try to do and you need to know how to handle them.
In any case keep in mind that a public proxy can be a honeypot and usually these are used for a lot of abuse by other people.