Volunteering and Observerships
Some residency programs consider any clinical experience that's not "hands-on" as insufficient. Other programs recognize any type of clinical experience even if it was just observerships.
In general, Americans have very high regards to volunteering. I know a friend who's got very low scores and no USCE but he was accepted in the specialty that he wants because he volunteered to the Red Crescent in his country.
Volunteering in a clinical environment (such as research volunteer) will take you high steps in the ranking of program directors, highly recommended.
As for your other question;
Observerships is usually free. You can get it by either looking up hospital websites and contacting attendings there or by connections with US doctors who can just personally take you there to shadow them.