I assume the original poster is inquiring about final year electives to get experience in a US hospital, not a US residency (at least at this time). Many US medical schools offer rotations to foreign visiting clerks, although some will not allow you to come for as long as 3 months. However, I met many German students doing 16 weeks at US schools so I know it is possible.
There exists no centralized service that I know of which list all the schools which take foreign clerks. Therefore you must check the schools individually - either by looking at their web sites (under Visiting Electives/International Students, etc.) or calling/writing them. You will be required to provide health and malpractice/liability insurance in effect in the US. Some schools will provide these for a fee if you do not have them - varies from school to school. Some charge an application fee or tuition to come for the elective but most do not.
Best of luck and I hope this helps.
