i get my parts where i can get the best deal with the safest transaction. i temper that with the need of how quickly the parts are required. if i need the part now, i get it from the local parts store. if i can wait a few days or a week, i get the best deal.
one thing though, deal only with sites that you trust. rockauto is good, as are most mustang parts suppliers, but there are fly by night places out there that will take your money and go out of business and you never get your parts.
my mind is aglow with whirling transient nodes of thought careening through a cosmic vapor of invention.