Merchants should contact their acquirer (merchant bank) or payment brand(s) directly to understand their PCI DSS validation requirements. See How do I contact the payment card brands? for information regarding contacting the payment brands.
Merchants wishing to engage a QSA for their PCI DSS review can find a list of QSAs on the PCI Council website - https://www.pcisecuritystandards.org/approved_companies_providers/qsa_companies.php
Article Number 1163