HCUPnet is a free, on-line query system that provides access to health statistics and information on hospital inpatient and emergency department utilization.
When high level data is insufficient, try searching the literature. Here are some PubMed suggestions to retrieve population studies.
Try adding the following MeSH terms to your PubMed search for a condition or disease:
add the "epidemiology" subheading to a MeSH term for a disease (example: Crohn Disease/epidemiology[MeSH])